Creation of the chromosomal theory of heredity. Chromosomal theory of heredity

Chapter 13. Genetics. The origin of the chromosomal theory of heredity. (V.N. Soifer)

Genetics - the science of heredity and its variability - developed at the beginning of the 20th century, after researchers paid attention to the laws of G. Mendel, discovered in 1865, but remained unnoticed for 35 years. In a short time, genetics has grown into a branched biological science with a wide range of experimental methods and directions. Its rapid development was determined both by the demands of agriculture, which needed a detailed development of the problems of heredity in plants and animals, and by the successes of biological disciplines, such as morphology, embryology, cytology, physiology and biochemistry, which prepared the ground for an in-depth study of the laws of heredity and material carriers hereditary factors. The name genetics was proposed for the new science by the English scientist W. Bateson in 1906.

Experiments on plant hybridization. Accumulation of information about inherited characteristics

Attempts to understand the nature of the transmission of traits by inheritance from parents to children were made in ancient times. Reflections on this topic are found in the writings of Hippocrates, Aristotle and other thinkers. In the 17th - 18th centuries, when biologists began to understand the process of fertilization and look for which principle - male or female - the secret of fertilization is connected with, debates about the nature of heredity resumed with renewed vigor. The famous struggle between preformationists (“animalculists” and “ovists”) greatly contributed to clarifying the nature of this process in animals. In plants, sexual differentiation was discovered by R. Ya. Cammerarius (1694), who discovered in experiments with spinach, hemp and corn that pollination is necessary for fruit set.

Thus, by the end of the 17th century. The scientific ground was prepared for the start of experiments on plant hybridization. The first successes in this direction were achieved at the beginning of the 18th century. It is believed that the first interspecific hybrid was obtained by the Englishman T. Fairchild by crossing the carnations Dianthus barbatus and D. caryophyllus. With the production of other hybrids, the practice of hybridization began to expand, but botanists still continued to consider the issue of the presence of two sexes in plants and their participation in fertilization controversial. In 1759, the St. Petersburg Academy of Sciences even announced a special competition to clarify this issue. The prize for his work “Study of sex in plants” (“Disquisitio de sexu plantarum”) was awarded in 1760 to C. Linnaeus, who obtained an interspecific hybrid of salsify (Tragopogon), which easily produces crosses in natural conditions. However, Linnaeus did not understand the essence of hybridization and the role of pollen in crossing. A scientifically based solution to this issue was achieved in the experiments of I. G. Kelreuter, a member of the Russian Academy of Sciences.

In 1760, Koelreuther began the first elaborate experiments to study the transmission of traits in plant crossings. In 1761 - 1766, almost a quarter of a century before L. Spallanzani, who studied the problem of crossing on animal objects, Kohlreuter, in experiments with tobacco, dope and cloves, showed that after the transfer of pollen of one plant to the pistil of another, plants differing in their morphological characteristics are formed ovaries and seeds that produce plants with properties intermediate in relation to both parents. As a result, Koelreuther came to a conclusion of fundamental importance: both parent organisms take part in the formation of offspring and the transmission of characteristics traceable in offspring. Koelreuter also introduced the method of backcrossing with one of the original parents, thanks to which he was able to prove the inheritance of traits and the equality of male and female elements in the formation of daughter individuals. The precise method of crossing developed by Koelreuther led to rapid progress in the study of hereditary transmission of traits.

At the end of the 18th - beginning of the 19th centuries. The English plant breeder T. E. Knight, while crossing various varieties, was faced with the problem of combining the characteristics of the parents in the descendants. Selecting different pairs for crossing, he discovered that each variety is characterized by a complex of small characteristics inherent in it. The number of characteristics in which two varieties differ from each other is greater, the lower the degree of their relationship. Knight's important conclusion was the discovery of the indivisibility of small characters in various crosses. The discreteness of hereditary material, proclaimed in ancient times, received the first scientific justification in his research. Knight is credited with the discovery of “elementary hereditary characteristics.”

Further significant advances in the development of the crossbreeding method are associated with the French school of breeders, especially with its most prominent representatives - O. Sajray and C. Naudin. The interests of both scientists were formed under the direct influence of Koelreuter and Knight. They took a step forward in terms of the selection of research objects, moving entirely to experiments with relatively quickly developing plants (vegetable crops), the vegetation cycle of which is limited to several months. Representatives of the pumpkin family became the favorite objects of Sajre and Naudin.

Sajre's greatest achievement was the discovery of the phenomenon of dominance. When crossing varieties that differed in hereditary characteristics, he often observed the suppression of the trait of one parent by the trait of the other. This phenomenon manifested itself to the maximum extent in the first generation after crossing, and then suppressed traits were again revealed in some of the descendants of subsequent generations. Thus, Sazhre confirmed that elementary hereditary characteristics do not disappear during crossings. Naudin came to the same conclusion quite independently in 1852 - 1869. But Naudin went even further, starting a quantitative study of the recombination of hereditary inclinations during crossings. Apparently, he was aware that it was precisely the quantitative description of the results of crossings that could provide researchers with the clue that would allow them to understand the essence of the processes that unfold during hybridization. However, Naudin was disappointed along this path. An incorrect methodological technique - the simultaneous study of a large number of signs - led to such confusion in the results that he was forced to abandon his attempt. The objects used by Naudin also introduced a considerable amount of uncertainty into the interpretation of the results obtained: he was not yet able to understand the role of self-pollinators in conducting such experiments. The shortcomings inherent in the experiments of Naudin and his predecessors were eliminated in the work of G. Mendel.

The development of the practice of hybridization led to the further accumulation of information about the nature of crosses. Important observations about combinations of traits in crosses began to accumulate as a result of the activities of gardeners and botanists. Practice required solving the problem of preserving the properties of “good” plants unchanged, as well as finding out ways to combine in one plant the necessary characteristics inherent in several parents. Similar tasks were set by livestock breeders, but invariably hung in the air, because they rested on ignorance of the laws of transmission of hereditary characteristics. It was not yet possible to solve this problem experimentally. Under such conditions, various speculative hypotheses about the nature of heredity arose.

Speculative hypotheses about the nature of heredity

The most fundamental hypothesis of this kind, which to a certain extent served as a model for similar constructions of other biologists, was the “temporary hypothesis of pangenesis” by Charles Darwin, set out in the last chapter of his work “Change in Domestic Animals and Cultivated Plants” (1868). Here Darwin summarized all the literature on crossings and the phenomena of heredity *.

* (Somewhat earlier, an analysis of the phenomena of heredity in humans was made by P. Luc in his extensive monograph “Traite philosophique et physiologique de l'heredite naturelle” (1847-1850).)

According to his ideas, in each cell of any organism, special particles are formed in large numbers - gemmules, which have the ability to spread throughout the body and collect (concentrate) in cells used for sexual or vegetative reproduction (eggs, sperm, plant buds). During fertilization, the gemmules of the two germ cells fuse to form a zygote. Some of the gemmules then give rise to new cells (similar to those from which they were formed), and some remain in an inactive state and can be passed on to subsequent generations. Darwin assumed that the gemmules of individual cells could change during the ontogenesis of each individual and give rise to modified descendants. Thus, he joined the supporters of the inheritance of acquired characteristics. In addition, he believed that since the complex of hereditary characteristics is composed of discrete factors of heredity (gemmules), then, consequently, the organism does not generate its own kind as a whole, but each individual unit generates its own similar one." *

* (C. Darwin. Soch., vol. 4. M., Publishing House of the USSR Academy of Sciences, 1951, p. 758.)

Darwin's assumption about the inheritance of acquired characteristics was experimentally refuted by F. Galton (1871). By undertaking a blood transfusion from black rabbits to white ones. Galton did not find any change in traits in the descendants. On this basis, he argued with Darwin, arguing that gemmules are concentrated only in the germ cells of plants and animals and the buds of vegetatively propagated plants and that the flow of gemmules from vegetative to generative parts does not occur. Galton resorted to an analogy, comparing the generative organs with the rhizomes of some plants, which produce new green shoots every year, from which his hypothesis received the name “rhizome hypothesis.”

A speculative hypothesis about the nature of heredity was proposed by the botanist K. Naegeli in his work “Mechanical and Physiological Theory of Evolution” (1884). Naegeli, thinking about the contradiction between the equal contribution of the father and mother to the formation of the offspring and the significantly different sizes of sperm and eggs, suggested that hereditary inclinations are transmitted only by part of the cell substance, which he called idioplasm. The rest (stereoplasm), according to his idea, does not carry hereditary characteristics. Naegeli also suggested that idioplasm consists of molecules connected to each other into large thread-like structures - micelles, grouped into bundles and forming a network that permeates all the cells of the body. The author did not know the facts confirming his model. During these years, attention had not yet been drawn to chromosomes as carriers of hereditary information, and Naegeli's hypothesis turned out to be, in a sense, prophetic. She prepared biologists to think about the structure of the material carriers of heredity. The hypothesis of intracellular pangenesis by G. de Vries was also famous.

For the first time, the idea of ​​differentiating (unequally hereditary) divisions of the nuclei of cells in a developing embryo was expressed by V. Roux in 1883. Roux’s conclusions had a great influence on A. Weissmann. They served as his starting point for creating the theory of germ plasm, which received its final form in 1892. Weisman clearly pointed to the carrier of hereditary factors - chromosomes. He believed that in the nuclei of cells there are special particles of germ plasm - biophores, each of which determines a separate property of the cells. Biophores, according to Weisman, are grouped into determinants - particles that determine the specialization of the cell. Since there are many different types of cells in the body, the determinants of one type are grouped into higher-order structures (ides), and the latter form chromosomes (or idants, in Weisman's terminology).

First, Roux (1883), and then Weisman, suggested the linear arrangement of hereditary factors (chromatin grains, according to Roux, and id, according to Weisman) in chromosomes and their longitudinal splitting during mitosis, which largely anticipated the future chromosomal theory of heredity.

Developing the idea of ​​unequal division, Weisman logically came to the conclusion that there are two clearly demarcated cell lines in the body - germinal (cells of the germinal pathway) and somatic. The former, ensuring the continuity of transmission of hereditary information, are “potentially immortal” and are capable of giving rise to a new organism. The latter do not have this property. The identification of two categories of cells had a great positive impact on the subsequent development of genetics. It, in particular, was the beginning of a theoretical refutation of the idea of ​​inheritance of acquired characteristics. At the same time, Weismann's theory of heredity also contained the erroneous assumption that the full set of determinants is contained only in germ cells.

The works of these biologists played an outstanding role in preparing scientific thought for the formation of genetics as a science. By the end of the 19th century. thanks to the work of cytologists who discovered chromosomes, studied mitotic (I. D. Chistyakov, 1872; A. Schneider, 1873; E. Strasburger, 1875; Schleicher, 1878; V. Flemming, 1892; etc.) and meiotic (E. van Beneden, 1883; T. Boveri, O. Hertwig, 1884) nuclear division, the ground was prepared for understanding the redistribution of hereditary material among daughter cells during their division. W. Waldeyer proposed the term chromosome in 1888. The process of fertilization in animals and plants was studied in detail (O. Hertwig, 1876; N.N. Gorozhankin, 1880; E. Strasburger, 1884; etc.). The work of botanists and livestock breeders paved the way for the rapid recognition of G. Mendel's laws after their rediscovery in 1900.

Discovery of the laws of inheritance by G. Mendel

The honor of discovering quantitative patterns accompanying the formation of hybrids belongs to the Czech amateur botanist Johann Gregor Mendel. In his works, carried out in the period from 1856 to 1863, the fundamentals of the laws of heredity were revealed.

Mendel formulated the problem of his research as follows. “Until now,” he noted in the “Introductory Remarks” to his work, “it has not been possible to establish a universal law of the formation and development of hybrids” and continued: “The final solution to this issue can only be achieved when detailed experiments are carried out in a variety of plant families. Whoever reconsiders the work in this area will be convinced that among the numerous experiments, not one was carried out in such a volume and in such a way that it was possible to determine the number of different forms in which the descendants of hybrids appear, and to reliably distribute these forms among individual generations and establish their mutual numerical relationships" *.

* (G. Mendel. Experiments on plant hybrids. M., "Science", 1965, pp. 9 - 10.)

The first thing Mendel paid attention to was the choice of object. For his research, Mendel chose the pea Pisum sativum L. The basis for this choice was, firstly, that the pea is a strict self-pollinator, and this sharply reduced the possibility of introducing unwanted foreign pollen; secondly, at that time there were a sufficient number of pea varieties that differed in one, two, three and four inherited traits.

Mendel received 34 varieties of peas from various seed farms. For two years, he checked whether the resulting varieties were not contaminated and whether they retained their characteristics unchanged when propagated without crossing. After this kind of verification, he selected 22 varieties for experiments.

Perhaps the most important thing in the whole work was determining the number of characteristics by which the crossed plants should be distinguished. Mendel first realized that only by starting with the simplest case - differences between parents on a single basis - and gradually increasing the complexity of the task, can one hope to unravel the tangle of facts. The strict mathematical nature of his thinking was revealed here with particular force. It was this approach to setting up experiments that allowed Mendel to clearly plan the further complexity of the initial data. He not only accurately determined which stage of work should be proceeded to, but also mathematically strictly predicted the future result. In this respect, Mendel stood above all contemporary biologists who studied the phenomena of heredity already in the 20th century.

Mendel began with experiments on crossing pea varieties that differed in one trait (monohybrid crossing). In all experiments without exception with 7 pairs of varieties, the phenomenon of dominance in the first generation of hybrids discovered by Sajre and Naudin was confirmed. Mendel introduced the concept of dominant and recessive traits, defining dominant traits that pass into hybrid plants completely unchanged or almost unchanged, and recessive traits that become hidden during hybridization. Then Mendel was for the first time able to quantify the frequencies of occurrence of recessive forms among the total number of descendants for cases of mono-, di-, tri-hybrid and more complex crosses. Mendel especially emphasized the average statistical nature of the pattern he discovered.

To further analyze the hereditary nature of the resulting hybrids, Mendel studied several more generations of hybrids crossed with each other. As a result, the following generalizations of fundamental importance received a solid scientific basis:

1. The phenomenon of inequality of hereditary elementary characters (dominant and recessive), noted by Sajray and Naudin.

2. The phenomenon of splitting the characteristics of hybrid organisms as a result of their subsequent crossings. Quantitative patterns of splitting were established.

3. Detection of not only quantitative patterns of splitting according to external, morphological characteristics, but also determination of the ratio of dominant and recessive inclinations among forms that are in appearance indistinguishable from dominant ones, but are mixed (heterozygous) in nature. Mendel confirmed the correctness of the last position, in addition, by backcrossing with parental forms.

Thus, Mendel came close to the problem of the relationship between hereditary inclinations (hereditary factors) and the characteristics of the organism determined by them.

The appearance of the organism (phenotype, according to the terminology of V. Johannsen, 1909) depends on the combination of hereditary inclinations (the sum of the hereditary inclinations of the organism began, according to Johannsen’s proposal, to be called the genotype, 1909). This conclusion, which inevitably followed from Mendel’s experiments, was discussed in detail by him in the section “Rudimentary cells of hybrids” of the same work “Experiments on plant hybrids”. Mendel was the first to clearly formulate the concept of a discrete hereditary inclination, independent in its manifestation from other inclinations *. These inclinations are concentrated, according to Mendel, in the rudimentary (egg) and pollen cells (gametes). Each gamete carries one deposit. During fertilization, the gametes fuse to form a zygote; Moreover, depending on the type of gametes, the zygote that arises from them will receive certain hereditary inclinations. Due to the recombination of inclinations during crossings, zygotes are formed that carry a new combination of inclinations, which determines the differences between individuals. This position formed the basis of Mendel's fundamental law - the law of gamete purity. His assumption about the presence of elementary hereditary inclinations - genes was confirmed by all subsequent development of genetics and was proven by research at different levels - organismal (using crossbreeding methods), subcellular (cytological methods) and molecular (physical and chemical methods). According to the proposal of W. Bateson (1902), organisms containing the same inclinations were called homozygous, and those containing different inclinations of the corresponding trait were called heterozygous for this trait.

* (Subsequently, these inclinations were called genes by V. Johannsen (1909).)

Experimental research and theoretical analysis of the results of crossings carried out by Mendel were ahead of the development of science by more than a quarter of a century. At that time almost nothing was known about the material carriers of heredity, the mechanisms of storage and transmission of genetic information and the internal content of the fertilization process. Even the speculative hypotheses about the nature of heredity discussed above were formulated later. This explains the fact that Mendel’s work did not receive any recognition in its time and remained unknown until the secondary rediscovery of Mendel’s laws by K. Correns, K. Cermak and G. de Vries in 1900.

Development of biometric methods for studying heredity

Individual differences, even between closely related organisms, are not necessarily due to differences in the genetic structure of these individuals; they may be caused by unequal living conditions. Therefore, conclusions about genetic differences between species, varieties, varieties and lines can only be made based on the analysis of a large number of individuals. The first to draw attention to mathematical patterns in individual variability was the Belgian mathematician and anthropologist A. Catlet. He was one of the founders of statistics and probability theory. Catlet paid special attention to the study of deviations in a series of similar individuals from the average quantitative characteristic of the trait being studied. However, in genetic terms, the most important question remained about the possibility of inheritance of deviations from the average quantitative characteristic of a trait observed in individual individuals. The significance of this issue became especially obvious after Darwin created the theory of natural selection. For purely practical purposes, it was necessary to find out whether and to what extent those individual changes that are often observed in breeding practice in individual plants would be inherited, and whether they could be fixed in the offspring.

Several researchers have begun to clarify this issue. The work of Galton, who collected data on the inheritance of height in humans, stood out in terms of its significance. He analyzed the heights of 204 married couples and 928 of their adult children. Galton then studied the inheritance of corolla size in sweet peas and came to the conclusion that only a small part of the deviations observed in the parents are transmitted to the offspring. Galton tried to give his observation a mathematical expression, thereby laying the foundation for a large series of works on the mathematical and statistical foundations of inheritance.

Galton's follower K. Pearson continued this work on a larger scale. A group of researchers quickly formed around Pearson and founded the journal Biometrics (1902).

The reasoning of English biometricians about the nature of the mixing of characteristics of parents during crossings, supported by mathematical calculations, but which, as a rule, did not take into account the biological essence of the phenomena of heredity, was dealt a blow by the secondary discovery of Mendel's laws. The most serious and classic study of the issues raised by Galton, Pearson and their followers was carried out in 1903 - 1909. V. Johannsen, who paid main attention to the study of genetically homogeneous material (offspring from inbreeding, called pure line by Johannsen). Johannsen's analysis allowed him to approach a true understanding of the role of heritable (genotypic) and non-heritable components in individual variation. Based on the results obtained, Johannsen gave a precise definition of genotype and phenotype and laid the foundations for modern understanding of the role of individual variability. Johannsen's conclusions, obtained in experiments with plants, were soon confirmed using zoological material.

Cytological foundations of genetics

Mendel's predictions were also confirmed at a completely different level of research. In the 70s - 80s of the XIX century. Mitosis and the behavior of chromosomes during cell division were described, which led to the idea. that these structures are responsible for the transfer of hereditary potencies from the mother cell to the daughter cells. The division of chromosome material into two equal parts was the best evidence in favor of the hypothesis that genetic memory is concentrated in chromosomes. This point of view was further strengthened by the description of the processes preceding the maturation of germ cells and fertilization (see Chapter 26). The study of chromosomes in animals and plants led to the conclusion that each species of living beings is characterized by a strictly defined number of chromosomes. This number has become a reliable systematic sign.

The fact discovered by E. van Beneden (1883) that the number of chromosomes in body cells (somatic cells) is twice as large as in germ cells could easily be explained by simple reasoning: since during fertilization the nuclei of germ cells merge (and, thus, in one the chromosomes of these nuclei are united in the nucleus) and since the number of chromosomes in somatic cells remains constant, the constant doubling of the number of chromosomes during successive fertilizations must be counteracted by a process leading to a reduction in their number in gametes by exactly half. An accurate description of the process of reduction division (meiosis), carried out in the 90s of the 19th century, made it possible already at the beginning of the 20th century. properly evaluate the patterns of heredity established by Mendel.

In 1900, independently of each other, three botanists - K. Correns in Germany, G. de Vries in Holland and E. Cermak in Austria discovered in their experiments the patterns previously discovered by Mendel and, having come across his work, published it again in 1901 This publication aroused deep interest in the quantitative laws of heredity. Cytologists discovered material structures whose role and behavior could be clearly related to Mendelian patterns. Such a connection was seen in 1903 by V. Setton, a young employee of the famous American cytologist E. Wilson. Mendel's hypothetical ideas about hereditary factors, the presence of a single set of factors in gametes and a double set in zygotes, were substantiated in studies of chromosomes. T. Boveri (1902) presented evidence in favor of the participation of chromosomes in the processes of hereditary transmission, showing that the normal development of a sea urchin is possible only if all chromosomes are present.

By establishing the fact that it is chromosomes that carry hereditary information, Satton and Boveri laid the foundation for a new direction in genetics - the chromosomal theory of heredity.

Rationale for the chromosomal theory of heredity

According to Mendel's laws, the manifestation of each hereditary factor does not depend on other factors. His analysis of mono-, di- and tri-hybrid crosses experimentally confirmed this conclusion.

After the rediscovery of Mendelian patterns, the study of these patterns began in all kinds of animal and plant species. One of the apparent failures befell W. Bateson and R. Punnett, who in 1906 studied the inheritance of corolla color and pollen shape in sweet peas. According to Mendel, the distribution of phenotypes in a dihybrid cross should obey the ratio 9:3:3:1. Instead, Batson and Punnett recorded a split ratio of 35:3:3:10. It seemed that the factors of purple color and wrinkled pollen tended to remain together during recombinations of inclinations. The authors called this phenomenon “mutual attraction of factors,” but they were unable to find out its nature.

In 1909, T. G. Morgan began a detailed study of this issue. First of all, he clearly formulated the initial hypothesis. Now that it was already known that hereditary inclinations are located in chromosomes, it was natural to answer the question: will the numerical patterns established by Mendel always be fulfilled? Mendel quite rightly believed that such patterns would be true if and only if the factors being studied were combined to form zygotes independently of each other. Now, on the basis of the chromosomal theory of heredity, it should be recognized that this is only possible when the genes are located on different chromosomes. But since the number of the latter is small compared to the number of genes, it would be expected that genes located on the same chromosome would pass from gametes to zygotes together. Consequently, the corresponding characteristics will be inherited in groups.

This assumption was tested by Morgan and his collaborators K. Bridges and A. Sturtevant in studies with the fruit fly Drosophila melanogaster. The choice of this object for many reasons can be considered a major success. Firstly, Drosophila has a very short development period (only 10 - 12 days); secondly, due to high fertility, it makes it possible to work with huge populations; thirdly, it can be easily cultivated in laboratory conditions; finally, she has only four pairs of chromosomes.

Soon, a large number of different mutations were discovered in Drosophila, that is, forms characterized by various hereditary characteristics. In normal or, as geneticists say, wild-type fruit flies, the body color is grayish-yellowish, the wings are gray, the eyes are dark brick-red, the bristles covering the body and the veins on the wings have a very specific arrangement. In mutant flies discovered from time to time, these characteristics were changed: the body, for example, was black, the eyes were white or otherwise colored, the wings were rudimentary, etc. Some individuals carried not one, but several mutations at once; for example, a fly with a black body could, in addition, have rudimentary wings. The variety of mutations allowed Morgan to begin genetic experiments. First of all, he proved that genes located on the same chromosome are transmitted jointly during crossings, that is, they are linked to each other. One linkage group of genes is located on one chromosome. Morgan also received strong confirmation of the hypothesis about the linkage of genes in chromosomes when studying the so-called sex-linked inheritance.

Thanks to cytological genetic experiments (A, Sturtevant, K. Bridges, G. J. Möller, 1910), it was possible to establish the participation of some chromosomes in sex determination. In Drosophila, for example, along with three pairs of chromosomes (autosomes) that are not related to sex determination, a pair of sex chromosomes was discovered. Sex chromosomes, in turn, turned out to be of two types - long rod-shaped X chromosomes and small curved Y chromosomes. Their combinations determine the sex of the fly. Further experiments showed that in Drosophila, as in most mammals (including humans), amphibians, fish and most plants, the entry of two X chromosomes into the zygote leads to the formation of a female individual, while the union of one X chromosome and one Y chromosome gives rise to a male individual *. Consequently, all female gametes are the same - they carry one X chromosome; males produce two types of gametes: half contain an X chromosome, half contain a Y chromosome. Therefore, during fertilization, half of the zygotes receive a set of chromosomes XX, and half - XY, and the sex ratio is 1:1.

* (In most birds, insects and some plants, sex determination occurs in a different way: male sex is obtained from the combination of two X chromosomes; the female sex is characterized by a combination of X and Y chromosomes)

By determining that the gene for Drosophila eye color is localized on the X chromosome, and by monitoring the behavior of the genes in the offspring of certain males and females, Morgan and his collaborators received convincing confirmation of the assumption of gene linkage.

Thus, there are two important stages in the development of genetics. The first, based on hybridological research, is associated with Mendel’s discovery - proof of the presence of elementary hereditary factors, establishing the nature of the interaction of these factors (the rule of dominance - recessivity) and elucidating quantitative patterns in the splitting of traits during crosses. The second stage, associated with the success of cytological research, ended with proof that chromosomes are carriers of hereditary factors. Morgan formulated and experimentally proved the concept of the linkage of genes in chromosomes. In particular, four linkage groups were discovered by genetic methods in Drosophila melanogaster, which coincided with the data of cytological studies. Next in line was the question of the order of genes in chromosomes.

The problem of intrachromosomal gene localization

A thorough analysis of the occurrence of mutations in Drosophila made it possible to detect a large number of diverse hereditary changes, and it turned out that each gene can give rise to a significant number of mutations. For example, mutants with red, white, purple, eosin, garnet, ivory, red, milky, and cinnabar eyes have been discovered. Other genes are characterized by similar variability.

As more and more new mutations were discovered, the volume of information about. localization of individual genes on one chromosome or another. The key to solving the question of the location of genes along the length of a chromosome was Morgan's study of the phenomena of disruption of gene linkage as a result of the exchange of sections between chromosomes (from one to several genes in length), which he called crossing over (in English, crossover).

An essential stage in the study of crossing over was the establishment of the fact that certain genes move from chromosome to chromosome with a certain frequency specific to them. Morgan suggested that the further apart the genes are located along the length of the chromosome, the easier crossing over can occur between them, because in order to separate closely lying genes, a gap must pass between them. The likelihood of such a gap is obviously low. And if this is so, then the percentage of individuals in which crossing over has occurred out of the total number of individuals studied can serve as a measure of the distance between genes on a chromosome. For his outstanding work in the field of genetics, Morgan was awarded the Nobel Prize in 1933.

In 1913, Sturtevant compiled the first map of the sex X chromosome of Drosophila, based on numerical data on linkage and crossing over observed in six sex-linked genes. By 1916, the chromosomal localization of hundreds of genes in Drosophila had already been studied, and they were mapped on all four chromosomes. The method for compiling genetic maps, developed in Drosophila, was transferred to plants (corn, snapdragon) and animals (mice).

Compiling genetic maps is a very labor-intensive procedure. The gene structures of chromosomes can be easily deciphered in those organisms that reproduce rapidly. The latter circumstance is the main reason that the most detailed maps exist for Drosophila, a number of bacteria and bacteriophages, and the least detailed for plants. Compiling maps for long-lived organisms (animals, perennial plants) is a matter of the future.

It should be noted that purely genetic methods for determining the localization of genes on chromosomes one way or another provided only indirect evidence of the chromosomal theory of heredity, and the latter continued to be challenged by some geneticists (for example, R. Goldschmidt, 1917). Direct evidence of this theory was the phenomenon of nondisjunction of sex chromosomes (1913, 1916) and loss of the fourth chromosome (1921) discovered by K. Bridges in Drosophila. In these cases, genetic predictions based on crosses were confirmed when karyotypes were examined under a microscope.

Finally, direct cytological evidence for the existence of crossing over in Drosophila was obtained. Back in 1909, the Belgian researcher F. Janssens came across an interesting fact. In the prophase of the first meiotic division, paired chromosomes approached each other, lined up in parallel, and then, touching their ends, quickly closed.

Despite the complete contact between the chromosomes of the salamanders with which Janssens worked, the outlines of each of the chromosomes were visible quite clearly. Thanks to this, it was possible to notice that during the twisting of chromosomes at the place of their interweaving, which he called the chiasma, an exchange of pieces of chromosomes occurred.

However, it was not possible to reliably confirm the presence of exchange using cytological methods until the German researcher K. Stern (1931) used the so-called phenomenon of translocation, i.e., the transfer of a broken piece of one chromosome to another chromosome. Using translocation, he managed to transfer a piece of the Drosophila Y chromosome to the X chromosome, after which the latter could easily be detected on cytological preparations. In addition, the resulting line of flies carried two genetic differences (their X chromosome had two easily detectable phenotypically so-called recessive marker genes).

The second stage of the work was the selection of a line of two flies with a translocation of a different kind. In this case, observations were made of an X chromosome that was torn in half, after which one of its halves was attached to a small Y chromosome. The remaining piece of the X chromosome was again clearly distinguishable both cytologically and genetically - its marking genes were dominant.

Thus, Stern had two lines of Drosophila, clearly distinguished from each other by X chromosomes. Having connected both marked X chromosomes in the zygote of one female, he waited for crossing over, recognizing it by the nature of the expression of genes. By cytologically analyzing the cells of the offspring of the crossover fly, he was able to detect the result of the crossing over in a visual form under the microscope: the long X chromosome had exchanged its large portion with a small piece of the short X chromosome, with the result that both chromosomes were now approximately the same length. Later, a similar experiment on corn was carried out by B. McClintock (1944).

Artificial mutations

The greatest achievement of experimental genetics was the discovery of the ability to artificially induce mutations using a variety of physical and chemical agents. G. A. Nadson and G. S. Filippov (1925) obtained mutations in yeast under the influence of radium and X-rays; G. Möller * (1927) - using X-rays in Drosophila, and L. Stadler (1928) - through exposure to the same rays in corn.

* (For the study of the phenomena of linkage and crossing over, as well as the discovery of artificial mutagenesis, G. Möller was awarded the Nobel Prize in 1946.)

A new, exceptionally fruitful period has begun in the study of the problem of variability. In a short period of time, the mutagenic effect of radiation was studied at many objects. It was found that under the influence of radiation, mutations of any type can occur. At the same time, to study the problem of the effect of radiant energy on biological systems, it was crucial to clarify the mutagenic activity of various types of radiation. It turned out that all known types of radiation are capable of causing hereditary changes. In the mid-30s, a theory was formulated that described the kinetic dependences of the inactivating and mutagenic effects of ionizing radiation - the so-called “target theory”. The most important experiments that became the basis of this theory were carried out in the period 1931 - 1937. N.V. Timofeev-Resovsky, M. Delbrück, R. Zimmer and other researchers.

An important achievement on the path to the artificial production of mutations was the work of V.V. Sakharov (1932, 1938) and M.E. Lobashev (1934, 1935) on chemical mutagenesis. Sakharov showed the mutagenic effect of iodine, and Lobashev - ammonium. A new stage in studying the role of chemical factors in the process of mutations was opened by I. A. Rapoport (1943, 1946, 1947) and S. Auerbach (1943), who pointed out the powerful mutagenic effect of some chemicals.

Currently, a large number of substances are known that enhance the mutation process. A theory of the action of mutagenic compounds on hereditary structures has been developed, and problems of the specificity of the action of mutagens are being intensively developed.

Classification of mutations

A large amount of material accumulated in the field of studying hereditary variability has made it possible to create a classification of types of mutations.

The existence of three classes of mutations was established - gene, chromosomal and genomic. The first class includes changes affecting only one gene. In this case, either the work of the gene is completely disrupted and, consequently, the body loses one of its functions, or its function changes. Chromosomal mutations, i.e. changes in the structure of chromosomes, in turn, are divided into several types. In addition to the translocations discussed above, doubling, tripling, etc. of individual sections of the chromosome can occur. Such mutations are called duplications. Sometimes a broken piece of a chromosome may remain on the same chromosome, but end up upside down; in this case, the order of genes in the chromosome changes. This type of mutation is called an inversion. If a section of a chromosome is lost, it is called a deletion, or deficiency. All these types of chromosomal rearrangements are combined under the general term - chromosomal aberrations.

Finally, mutations can be expressed in changes in the number of chromosomes. Such mutations are called genomic. It turned out that individual chromosomes can be doubled or lost, resulting in the formation of heteroploids. More often, the set of chromosomes increases multiple times and polyploids arise, that is, cells or entire organisms with redundant sets of chromosomes.

The study of chromosome sets (karyotypes) of various species has revealed the widespread occurrence of polyploidy in nature, especially among plants, for many of which a large number of polyploid series have been described. For example, representatives of the genus Triticum are arranged in the following row - Triticum toposossitis has 14 chromosomes (diploids); Tr. turgidum, Tr. durum carry 28 chromosomes (tetraploid); at Tr. vulgare and Tr. spelta, the number of chromosomes is 42 (hexaploids). In the genus Solanum the following series has been traced: 12, 24, 36, 48, 60, 72, 96, 108, 144 chromosomes (the haploid number of chromosomes in this genus can be multiplied up to 24 times). The genus Rosa is characterized by the following: 14, 21, 28, 35, 42, 56 chromosomes. Polyploid series do not necessarily contain members with doubled, quadrupled, sixfold, etc. sets of chromosomes. Thus, in the genus Crepis there is clearly defined polyploidy, but the number of chromosomes in a row increases as follows: 6, 8, 10, 12, 16, 18, 24, 40, 42. There are many such genera in the plant kingdom.

Artificial production of polyploids

After the discovery of natural polyploids, it was possible to artificially obtain polyploids of various organisms. This discovery was the most important achievement of experimental genetics.

One of the first artificial polyploids were tomatoes and nightshade with quadruple sets of chromosomes, obtained by G. Winkler in 1916. With the discovery of polyploidogenic substances (the alkaloid colchicine, a petroleum sublimation product - acetanaphthene, etc.), it became possible to unusually speed up the production of polyploids and, on their basis, begin selection of new, high-yielding plant varieties.

In 1927, G.D. Karpechenko, using the polyploidy method, for the first time in the world, created a new organism not found in nature, called Raphanobrassica, in which the chromosomes of radish (Raphanus) were combined with the chromosomes of cabbage (Brassica). Depending on the content of chromosomes of one kind or another in the cells of the new plant, the shape of its fruit changed. So, with an equal number of both chromosomes, the fruit was half rare, half cabbage; with a combination of 9 rare and 18 cabbage chromosomes, it was two-thirds cabbage and one-third rare, etc. Assessing his work, Karpechenko noted that it can be considered as an experimental substantiation of the theory of the hybrid origin of polyploid species. The Swedish geneticist A. Müntzing (1930), using the method of crossings, managed to obtain a third - 32-chromosome - G. tetrahit (1932) from two 16-chromosomal species of pickleweed (Galeopsis speciosa, G. pubescens).

It was later discovered that polyploidy is not limited to the plant world. Using the same method of polyploidization, B.L. Astaurov achieved in the 40s the production of fertile hybrids by crossing silkworms of two species, Bombuch mori and B. mandarina.

Studying the genetic basis of evolution

The proof of the proposition that recessive traits are inextinguishable when crossing organisms, put forward by Mendel, turned out to be very important for the development of evolutionary teaching. This position made it possible to overcome the objection expressed by the English mathematician F. Jenkin that newly emerging hereditary changes in nature cannot spread in nature due to “dissolution” among the surrounding mass of normal, unchanged individuals. After the rediscovery of Mendel's laws and proof that the factors determining the development of heritable characteristics are passed on to descendants without fragmentation, the “Jenkip nightmare” was dispelled. It became clear that all mutations that occur naturally do not disappear, but pass either into a recessive state or remain dominant (see also Chapter 17).

In 1904, K. Pearson substantiated the so-called law of stabilizing crossing, according to which, under conditions of free crossing, at any initial ratio of the numbers of homozygous and heterozygous parental forms, as a result of the first crossing, a state of equilibrium is established within the community. In 1908, the English mathematician G. Hardy came to the conclusion that in unlimitedly large populations, in the presence of free crossing, in the absence of mutation pressure, migration and selection, the relative numbers of homozygous (both dominant and recessive) and heterozygous individuals will remain constant, provided the product of the number of homozygous (dominant on recessive) individuals is equal to the square of half the number of heterozygous forms. Thus, according to Hardy’s law (often also called the Hardy-Weiberg law), in a population in the presence of free crossing there should be a completely definite and equilibrium-maintained distribution of mutant forms. It should be emphasized that although the mathematically strict form of these patterns gave a very clear idea of ​​the genetic basis of the evolutionary process, these patterns were not recognized by evolutionary biologists for a long time. There was a gap between Darwinism and genetics, and work in one area was carried out in complete isolation from work in the other.

Only in 1926 did S.S. Chetverikov publish a large work that for the first time drew attention to the general biological significance of the calculations of Pearson, Hardy and others. Chetverikov examined in detail the biological and genetic foundations of evolution (the role of mutations, or genovariations, in his terminology, the spread of mutations in conditions of free crossing, the role of natural selection and isolation, the role of the genotypic environment) and laid the foundations of a new scientific discipline - population genetics. The further development of population genetics was associated with the works of S. Wright, R. Fisher, N. P. Dubinin, F. G. Dobzhansky and others.

Chetverikov and his students N.K. Belyaev, S.M. Gershenzon, P.F. Rokitsky and D.D. Romashov were the first to carry out an experimental genetic analysis of natural populations of Drosophila, which fully confirmed their saturation with recessive mutations. Similar results were obtained by E. A. and N. V. Timofeev-Resovsky when studying Drosophila populations (1927 - 1931), as well as by other researchers.

Chetverikov's ideas served as the basis for further study of population genetics. The patterns derived by Pearson and Hardy were valid only for “ideal” populations. Subsequent analysis of the conclusions of these authors showed that they are applicable only to an abstract, unlimited population; in real populations, there is a deviation of the actual frequency of mutation retention from the expected one. This process is carried out according to probabilistic laws and leads to a sharp restructuring of the genetic structure of the population. Since out of all the offspring of any pair of parents, only two individuals reach sexual maturity and give offspring on average, the possibility of maintaining a newly emerged mutation in the population depends on many reasons (the probability of its death; the frequency of re-occurrence of the same mutation; differences in the number of descendants remaining from different parents, degree of isolation in the population, etc.).

It was found that the preservation and spread of mutations in a population is determined by genetic-automatic processes. A detailed analysis of these processes was carried out by Romashov (1931), Dubinin (1931) and Wright (1921, 1931). The latter called them “the phenomenon of genetic drift in a population,” and Chetverikov called them “genetic-stochastic,” emphasizing their probabilistic-statistical nature. Statistical analysis, supported by experiments in real populations, showed that on average, out of 104 different simultaneously occurring mutations, after 100 generations about 150 mutations remain, and after 500 generations - only 40 *. Thus, as a result of genetic-automatic processes, many emerging mutations are destroyed and only a few are brought to the level of noticeable concentrations. Since selection in a population strongly depends on the average concentrations of alleles, an increase in the number of individual mutations due to genetic-automatic processes should lead to a sharp increase in the rate of selection in the population. Due to the probabilistic nature of genetic-automatic processes, they can either eliminate individual mutations or increase their numbers, allowing selection to carry out the “trial and error” mechanism. Genetic-automatic processes constantly bring rare mutations to the level of selection and thereby help the latter to quickly “reconsider” new variants of mutants. If selection rejects mutations, they quickly move to a zone of low concentrations or completely disappear from the population; if selection picks them up, they quickly spread through the population, bypassing the long phase of remaining in low concentration, inaccessible to selection. Thus, genetic-automatic processes accelerate the evolution of new mutations by reducing the early stages of reproduction of newly emerged mutations.

* (I. P. Dubinin. Evolution of populations and radiation. M., Atomizdat, 1966.)

A detailed study of the genetic structure of natural populations and the rate of spread of mutations in nature has now become a field of biology, actively developed on the basis of mathematical methods. Of great importance for the development of this field are model experiments in which the fate of experimentally created populations is studied and the role of various forms of isolation and selection is determined.

The problem of gene fragmentation

By the beginning of the 30s of the XX century. The foundations of the gene theory were formed. Already the first achievements of hybridological analysis raised the problem of discreteness of hereditary material. In Mendel's experiments, this idea received reliable experimental confirmation. It was believed that the gene was responsible for the development of one trait and was transmitted during crossings as an indivisible whole. The discovery of mutations and crossing over initially also confirmed the indivisibility of genes. Thus, A. Katell obtained other mutants from mutant (yellow) fruit flies, but any new mutation captured the entire gene. N.V. Timofeev-Resovsky (1925-1929), G. Möller (1928) and M. Demerets (1928), having received so-called reverse mutations (i.e., turning mutant flies into normal ones), made sure that one state of the gene completely replaced by a new one. When studying crossing over, it was also found that during this process pieces of chromosomes of different lengths can be transmitted, but the minimum transmitted region corresponds to one gene. Gaps within the gene were never observed. As a result of generalizing all these data, the definition of a gene received the following formulation: a gene is an elementary unit of heredity, characterized by a very specific function, mutating during crossing over as a whole. In other words, a gene is a unit of genetic function, mutation and crossing over.

In 1928, this seemingly well-established theory of the indivisibility of the gene underwent its first limitation. Immediately after the discovery of the mutagenic effect of X-rays, they were used in many laboratories around the world to produce mutations. Such work was also carried out in the laboratory of A. S. Serebrovsky at the Biological Institute. K. A. Timiryazeva. In 1928, in the same laboratory, N.P. Dubinin began to study the effect of X-rays on fruit flies and discovered an unusual mutation. The formation of bristles on the body of flies is controlled by a special scute gene. The mutation of the scute gene, first discovered by the American geneticist Paine (1920), appeared more than once in experiments, and when it appeared, the development of nine setae was suppressed. The scute mutation identified by Dubinin suppressed the development of only four setae. Since the generally accepted concept of a complete mutation of a gene, the appearance of such a mutation seemed completely incomprehensible. In the next experiment, a mutation was found that affected not 4 or 9, but 18 bristles on the fly’s body. In other words, it was as if two genes were damaged at once. Dubinin designated these mutations with the symbols scute-1, scute-2 and scute-Z. It became clear that a gene is not an indivisible genetic structure, but is a region of a chromosome, individual sections of which can mutate independently of each other. This phenomenon was called Serebrovsky step allelomorphism.

Following N.P. Dubinin, I.I. Agol found a fourth mutation - scute-4, which did not coincide with the first three; A. E. Gaisinovich - scute-5; then A.S. Serebrovsky discovered the scute-b mutation; S. G. Levit - scute-7; B. N. Sidorov - scute-8; N. P. Dubinin - mutations scute-9, scute-10, scute-11, scute-13, scute-15, scute-16, scute-17; H. I. Shapiro - scute-12; L.V. Ferry - scute-14. Thus, the phenomenon of gene fragmentation was finally proven.

One of the major advantages of the work on the study of stepwise allelomorphs was the quantitative method of accounting for mutants. Having developed a system that made it possible to quantitatively evaluate the result of each mutation, Serebrovsky, Dubinin and other authors then discovered the phenomenon of complementing one mutant gene with another. In this case, the impaired function of one gene was corrected by the normal function of another gene. The second gene, in turn, could be defective in another region that was normal in the first gene. This phenomenon was subsequently rediscovered in microorganisms and was called complementation. For a series of works on the chromosomal theory of heredity and the theory of mutations, Dubinin was awarded the Lenin Prize in 1966.

Having demonstrated the mutational fragmentation of the gene, Serebrovsky and the staff of his laboratory, however, for a long time could not confirm the fragmentation of the gene using crossing over. The fact is that the resolution of crossing over in relation to the chromosomes of higher organisms is very limited. To detect a gene break, it was necessary to test a huge number of flies. It was possible to organize such an experiment only in 1938, when N.P. Dubinin, N.N. Sokolov and G.G. Tinyakov were able to break the scute gene and check their result cytologically on the giant chromosomes of the salivary glands of Drosophila. The final solution to the question of whether a gene is divisible not only mutationally, but also mechanically, was achieved in the works of M. Green (1949), E. Lewis (1951) and G. Pontecorvo (1952). It was finally established that it is incorrect to consider the gene as an unusually stable, further indivisible structure. The time has come to develop a new gene theory, to identify specific physical structures responsible for the implementation of various genetic functions. It was not possible to solve these problems in complex multicellular organisms due to purely technical difficulties, because for this it was necessary to study tens and hundreds of thousands of flies. Microorganisms came to the rescue.

The transition to genetic research on microorganisms was the largest step forward in the study of genetic problems. The new research objects had the advantage that they produced huge populations, multiplied extremely quickly, had an extremely simple genetic apparatus (their chromosomes consisted of a single DNA molecule), and had clear, well-selectable mutants. With the development of experiments on microorganisms, genetics moved to the molecular level of research, which brought answers to many mysteries of the organization of living things.

The chromosomal theory of heredity is based on scientists' knowledge of the structure of genes and their transmission to next generations. This makes it possible to answer some questions related to our origin, external data, behavior, diseases, etc. The chromosomal theory of heredity consists in the order of transmission from parents to children of information found in genes, which together give rise to a new person.

Heredity

Information is inherited through thousands of genes that are located in the nuclei of the egg and sperm, forming a new organism. Each gene has a code that synthesizes one specific type of protein. This process is orderly, which makes it possible to predict the characteristics of the future generation. This is explained by the fact that genes (units of inheritance) are combined in a certain order. An interesting fact remains that each cell contains a pair of chromosomes responsible for one protein. Thus, each gene is paired (allelic). One of them dominates, the other is in a “sleeping” state. This is inherent in all cells of the body, except for the sex cells (those have only one strand of DNA in order to form a full-fledged nucleus with a full set of chromosomes during fusion into a zygote). These simple truths are called the “chromosomal theory of heredity”, or Mendelian genetics.

Offspring

During the formation of gametes, pairs of genes separate, but during fertilization, something else happens: the genes of the egg and sperm are combined. The new combination makes it possible to identify the development of certain characteristics in offspring. Since each parent has allelic genes, they cannot predict which ones will be passed on to the child. Of course, according to one of Mendel’s laws, dominant genes are stronger, and therefore there is a high probability that they will manifest themselves in a child, but it all depends on the case.

Diseases

Human chromosomes are made up of 23 pairs. Sometimes the set may be incorrect as a result of the attachment of an extra gene. Then various kinds of mutations can arise. This is also called “chromosomal syndrome” - a change in the structure of the DNA chain: inversion of a chromosome, its loss, duplication, rearrangement in a certain area. It is also possible to exchange sections of dissimilar chromosomes, rearrange a certain section, or transfer a gene from one chromosome to another. The following diseases are striking examples of such manifestations.

1. Cry of the cat syndrome

The chromosomal theory of heredity confirms that such a disorder is caused by the loss of the short arm of the fifth chromosome. This disease manifests itself in the first minutes of life in the form of crying, similar to a cat’s “meow.” After several weeks, this symptom disappears. The older the child, the more abnormal development is visible: at first he is distinguished by low weight, then facial asymmetry is more and more clearly noticeable, microcephaly appears, slanted eyes, the bridge of the nose is wide, abnormal ears with an external auditory canal, and a possible heart defect. Physical and mental retardation is an integral part of the disease.

• Aneuploidy(the number of chromosomes not a multiple of the haploid set). A striking example is Edwards syndrome. Manifested by early birth, the fetus has hypoplasia of the skeletal muscles, low weight, and microcephaly. The presence of a cleft lip, the absence of a big toe, defects of internal organs, and their abnormal development are determined. Only a few survive and remain mentally retarded throughout their lives.
• Polyploidy(multiple number of chromosomes). Patau syndrome is manifested by external and mental abnormalities. Children are born deaf and have mental retardation. The chromosomal theory of heredity is always confirmed, which makes it possible to predict the development of the fetus in the womb and, if necessary, terminate the pregnancy.

Topic 32. Chromosomal theory of heredity. Morgan's Law

Introduction
1. T. G. Morgan - the greatest geneticist of the 20th century.
2. Attraction and repulsion
3. Chromosomal theory of heredity
4. Mutual arrangement of genes
5. Maps of linkage groups, localization of genes in chromosomes
6. Cytological maps of chromosomes
7. Conclusion
Bibliography

1. INTRODUCTION

Mendel's third law - the rule of independent inheritance of characters - has significant limitations.
In Mendel's own experiments and in the first experiments carried out after the second discovery of Mendel's laws, genes located on different chromosomes were included in the study, and as a result, no discrepancies with Mendel's third law were found. Somewhat later, facts were found that contradict this law. The gradual accumulation and study of them led to the establishment of the fourth law of heredity, called Morgan's law (in honor of the American geneticist Thomas Gent Morgan, who first formulated and substantiated it), or the rule of linkage.
In 1911, in the article “Free segregation as opposed to attraction in Mendelian heredity,” Morgan wrote: “Instead of free segregation in the Mendelian sense, we found an “association of factors” localized close together on the chromosomes. Cytology provided the mechanism required by the experimental data.
These words briefly formulate the main provisions of the chromosomal theory of heredity developed by T. G. Morgan.

1. T. G. MORGAN - THE LARGEST GENETICIST of the 20th century.

Thomas Gent Morgan was born on September 25, 1866 in Kentucky (USA). In 1886 he was graduated from the university of this state. In 1890, T. Morgan received his Doctor of Philosophy degree, and the following year became a professor at a women's college in Pennsylvania. The main period of his life was associated with Columbia University, where from 1904 for 25 years he served as head of the department of experimental zoology. In 1928, he was invited to head a biological laboratory specially built for him at the California Institute of Technology, in a town near Los Angeles, where he worked until his death.
T. Morgan's first studies were devoted to issues of experimental embryology.
In 1902, the young American cytologist Walter Setton (1877-1916), who worked in the laboratory of E. Wilson (1856-1939), suggested that the peculiar phenomena characterizing the behavior of chromosomes during fertilization were, in all likelihood, a mechanism of Mendelian patterns . T. Morgan was well acquainted with E. Wilson himself and with the work of his laboratory, and therefore, when in 1908 he established in male phylloxera the presence of two varieties of sperm, one of which had an additional chromosome, an assumption of a connection immediately arose characteristics of sex with the introduction of appropriate chromosomes. So T. Morgan moved on to the problems of genetics. He came up with the idea that not only gender is associated with chromosomes, but, perhaps, other hereditary inclinations are localized in them.
The modest budget of the university laboratory forced T. Morgan to search for a more suitable object for experiments in the study of heredity. From mice and rats he moves on to the fruit fly Drosophila, the choice of which turned out to be extremely successful. The work of T. Morgan's school, and then most other genetic research institutions, focused on this object. Major discoveries in genetics of the 20-30s. XX century associated with Drosophila.
In 1910, T. Morgan’s first genetic work, “Sex-Limited Heredity in Drosophila,” was published, describing the white-eyed mutation. The subsequent, truly gigantic work of T. Morgan and his colleagues made it possible to link the data of cytology and genetics into a single whole and culminated in the creation of the chromosomal theory of heredity. The major works of T. Morgan “Structural basis of heredity”, “Gene theory”, “Experimental foundations of evolution” and others mark the progressive development of genetic science.
Among biologists of the twentieth century. T. Morgan stands out as a brilliant experimental geneticist and as a researcher of a wide range of issues.
In 1931, T. Morgan was elected an honorary member of the USSR Academy of Sciences, and in 1933 he was awarded the Nobel Prize.

2. ATTRACTION AND REPULSION

For the first time, a deviation from the rule of independent inheritance of characters was noticed by Bateson and Punnett in 1906 when studying the nature of inheritance of flower color and pollen shape in sweet peas. In sweet pea, purple flower color (controlled by the B gene) is dominant over red (depending on gene B), and the oblong shape of mature pollen (“long pollen”), associated with the presence of 3 pores, which is controlled by the L gene, dominates “round” pollen with 2 pores, the formation of which is controlled by the l gene.
When crossing purple sweet peas with long pollen and red sweet peas with round pollen, all first generation plants have purple flowers and long pollen.
In the second generation, among the 6,952 plants studied, 4,831 plants with purple flowers and long pollen, 390 with purple flowers and round pollen, 393 with red flowers and long pollen, and 1,338 with red flowers and round pollen were found.
This ratio corresponds well to the splitting that is expected if, during the formation of gametes of the first generation, genes B and L are found 7 times more often in the combinations in which they were found in the parental forms (BL and bl) than in new combinations (Bl and bL) (Table 1).
It seems that genes B and L, as well as b and l, are attracted to each other and can only be separated from one another with difficulty. This behavior of genes was called gene attraction. The assumption that gametes with the B and L genes in the combinations in which they were presented in the parental forms are found 7 times more often than gametes with a new combination (in this case Bl and bL) was directly confirmed in the results as called analyzing crosses.
When crossing first generation (F1) hybrids (genotype BbLl) with a recessive parent (bbll), the following split was obtained: 50 plants with purple flowers and long pollen, 7 plants with purple flowers and round pollen, 8 plants with red flowers and long pollen, and 47 plants with red flowers and round pollen, which corresponds very well to the expected ratio: 7 gametes with old gene combinations to 1 gamete with new combinations.
In those crosses where one of the parents had the BBll genotype and the other the bbLL genotype, segregation in the second generation had a completely different character. In one of these F2 crosses, there were 226 plants with purple flowers and long pollen, 95 with purple flowers and round pollen, 97 with red flowers and long pollen, and one plant with red flowers and round pollen. In this case, it appears that the B and L genes repel each other. This behavior of hereditary factors was called gene repulsion.
Since the attraction and repulsion of genes was very rare, it was considered some kind of anomaly and a kind of genetic curiosity.
Somewhat later, several more cases of attraction and repulsion were discovered in sweet peas (flower shape and leaf axil color, flower color and flower sail shape, and some other pairs of characters), but this did not change the overall assessment of the phenomenon of attraction and repulsion as an anomaly.
However, the assessment of this phenomenon changed dramatically after in 1910-1911. T. Morgan and his students discovered numerous cases of attraction and repulsion in the fruit fly Drosophila, a very favorable object for genetic research: its cultivation is cheap and can be carried out in laboratory conditions on a very wide scale, its lifespan is short and in one year you can get several dozen generations, controlled crossings are easy to implement; there are only 4 pairs of chromosomes, including a pair of sexual ones that are clearly distinguishable from each other.
Thanks to this, Morgan and his colleagues quickly discovered a large number of mutations in hereditary factors that determine traits that are clearly visible and easy to study, and were able to conduct numerous crosses to study the nature of inheritance of these traits. It turned out that many genes in the Drosophila fly are not inherited independently of each other, but are mutually attracted or repelled, and genes showing such interaction could be divided into several groups, within which all genes showed more or less strongly expressed mutual attraction or repulsion.
Based on an analysis of the results of these studies, T. G. Morgan suggested that attraction occurs between non-allelomorphic genes located on the same chromosome and persists until these genes are separated from each other as a result of chromosome breakage during reduction division , and repulsion occurs in cases where the genes being studied are located on different chromosomes of the same pair of homologous chromosomes
It follows that the attraction and repulsion of genes are different aspects of the same process, the material basis of which is the different arrangement of genes in the chromosomes. Therefore, Morgan proposed to abandon the two separate concepts of “attraction” and “repulsion” of genes and replace it with one general concept of “gene linkage,” believing that it depends on their location within one chromosome in a linear order.

3. CHROMOSOMAL THEORY OF HERITAGE

Upon further study of gene linkage, it was soon established that the number of linkage groups in Drosophila (4 groups) corresponds to the haploid number of chromosomes in this fly, and all genes studied in sufficient detail were distributed among these 4 linkage groups. Initially, the relative location of genes within a chromosome remained unknown, but later a technique was developed to determine the order of location of genes included in the same linkage group, based on the quantitative determination of the strength of linkage between them.
Quantitative determination of gene linkage strength is based on the following theoretical premises. If two genes A and B in a diploid organism are located on one chromosome, and recessive allelomorphs of these genes a and b are located on another chromosome homologous to it, then genes A and B can separate from each other and enter into new combinations with their recessive allelomorphs only in in the event that the chromosome in which they are located is broken in the area between these genes and at the site of the break a connection occurs between sections of this chromosome and its homologue.
Such breaks and new combinations of chromosome regions actually occur during the conjugation of homologous chromosomes during reduction division. But in this case, exchanges of sections usually do not occur between all 4 chromatids that make up the chromosomes of bivalents, but only between two of these 4 chromatids. Therefore, the chromosomes formed as a result of the first division of meiosis, during such exchanges, consist of two unequal chromatids - unchanged and reconstructed as a result of the exchange. In the II division of meiosis, these unequal chromatids diverge to opposite poles, and thanks to this, haploid cells resulting from reduction division (spores or gametes) receive chromosomes consisting of identical chromatids, but only half of the haploid cells receive reconstructed chromosomes, and the second half receive unchanged.
This exchange of chromosome sections is called crossing over. All other things being equal, crossing over between two genes located on the same chromosome occurs less frequently the closer they are located to each other. The frequency of crossing over between genes is proportional to the distance between them.
Determining the frequency of crossing over is usually done using so-called analytical crosses (crossing F1 hybrids with a recessive parent), although F2 obtained from selfing of F1 hybrids or crossing F1 hybrids with each other can also be used for this purpose.
We can consider this determination of the frequency of crossing over using the example of the strength of adhesion between the C and S genes in maize. The C gene determines the formation of colored endosperm (colored seeds), and its recessive allele c causes uncolored endosperm. The S gene causes the formation of smooth endosperm, and its recessive allele s determines the formation of wrinkled endosperm. Genes C and S are located on the same chromosome and are quite strongly linked to each other. In one of the experiments conducted to quantify the strength of adhesion of these genes, the following results were obtained.
A plant with colored smooth seeds, homozygous for the C and S genes and having the CCSS genotype (dominant parent), was crossed with a plant with uncolored wrinkled seeds with the CCSS genotype (recessive parent). First generation F1 hybrids were recrossed to the recessive parent (test cross). In this way, 8368 F2 seeds were obtained, in which the following splitting was found based on color and wrinkles: 4032 colored smooth seeds; 149 painted wrinkled; 152 unpainted smooth; 4035 undyed wrinkled.
If, during the formation of macro- and microspores in F1 hybrids, the C and S genes were distributed independently of each other, then in the testing cross all these four groups of seeds should be represented in equal numbers. But this is not the case, since the C and S genes are located on the same chromosome, linked to each other, and as a result, disputes with recombined chromosomes containing the Cs and cS genes are formed only in the presence of crossing over between the C and S genes, which occurs relatively rare.
The percentage of crossing over between genes C and S can be calculated using the formula:

X = a + b / n x 100%,

Where a is the number of crossing over grains of one class (grains with the Cscs genotype, derived from the combination of gametes Cs of the F1 hybrid with gametes cs of the recessive parent); c is the number of crossing-over grains of the second class (cScs); n is the total number of grains obtained as a result of analyzing crossing.
Diagram showing the inheritance of chromosomes containing linked genes in maize (according to Hutchinson). The hereditary behavior of the genes for colored (C) and colorless (c) aleurone, full (S) and wrinkled (s) endosperm, as well as the chromosomes carrying these genes when crossing two pure types with each other and when backcrossing F1 with a double recessive is indicated.
Substituting the number of grains of different classes obtained in this experiment into the formula, we obtain:

X = a + b / n x 100% = 149 + 152 / 8368 x 100% = 3.6%

The distance between genes in linkage groups is usually expressed as a percentage of crossing over, or in morganids (a morganid is a unit expressing the strength of linkage, named at the suggestion of A. S. Serebrovsky in honor of T. G. Morgan, equal to 1% crossing over). In this case, we can say that the C gene is located at a distance of 3.6 morganids from the S gene.
Now you can use this formula to determine the distance between B and L in sweet peas. Substituting the numbers obtained from analytical crossing and given above into the formula, we get:

X = a + b / n x 100% = 7 + 8 / 112 x 100% = 11.6%

In sweet peas, genes B and L are located on the same chromosome at a distance of 11.6 morganids from each other.
In the same way, T. G. Morgan and his students determined the percentage of crossing over between many genes included in the same linkage group for all four Drosophila linkage groups. It turned out that the percentage of crossing over (or the distance in morganids) between different genes that are part of the same linkage group turned out to be sharply different. Along with genes between which crossing over occurred very rarely (about 0.1%), there were also genes between which linkage was not detected at all, which indicated that some genes are located very close to each other, while others are very close to each other. far.

4. RELATIVE LOCATION OF GENES

To figure out the location of genes, it was assumed that they were arranged in a linear order on chromosomes and that the true distance between two genes was proportional to the frequency of crossing over between them. These assumptions opened up the possibility of determining the relative position of genes within linkage groups.
Suppose the distances (% crossing over) between three genes A, B and C are known and that they are 5% between genes A and B, 3% between B and C and 8% between genes A and C.
Let's assume that gene B is located to the right of gene A. In which direction from gene B should gene C be located?
If we assume that gene C is located to the left of gene B, then in this case the distance between gene A and C should be equal to the difference in the distances between genes A - B and B - C, i.e. 5% - 3% = 2%. But in reality, the distance between genes A and C is completely different and is equal to 8%. Therefore the assumption is incorrect.
If we now assume that gene C is located to the right of gene B, then in this case the distance between genes A and C should be equal to the sum of the distances between genes A - B and genes B - C, i.e. 5% + 3% = 8 %, which fully corresponds to the distance established experimentally. Therefore, this assumption is correct, and the location of genes A, B and C on the chromosome can be schematically depicted as follows: A - 5%, B - 3%, C - 8%.
Once the relative positions of the 3 genes have been established, the location of the fourth gene in relation to these three can be determined by knowing its distance from only 2 of these genes. We can assume that the distance of gene D from two genes - B and C from among the 3 genes A, B and C discussed above is known and that it is equal to 2% between genes C and D and 5% between B and D. An attempt to place gene D on the left from gene C is unsuccessful due to the obvious discrepancy between the difference in distances between genes B - C and C - D (3% - 2% = 1%) to the given distance between genes B and D (5%). And, on the contrary, placing gene D to the right of gene C gives complete correspondence between the sum of the distances between genes B - C and genes C - D (3% + 2% = 5%) to the given distance between genes B and D (5%). Once we have established the location of gene D relative to genes B and C, without additional experiments we can calculate the distance between genes A and D, since it should be equal to the sum of the distances between genes A - B and B - D (5% + 5 % = 10%).
When studying the linkage between genes included in the same linkage group, an experimental check of the distances between them, previously calculated in the same way as was done above for genes A and D, was repeatedly carried out, and in all cases a very good agreement was obtained.
If the location of 4 genes is known, say A, B, C, D, then the fifth gene can be “linked” to them if the distances between gene E and some two of these 4 genes are known, and the distances between gene E and the other two genes quadruples can be calculated as was done for genes A and D in the previous example.

5. MAPS OF LINKAGE GROUPS, LOCALIZATION OF GENES IN CHROMOSOMES

By gradually linking more and more genes to the original three or four linked genes, for which their relative positions had previously been established, maps of linkage groups were compiled.
When compiling clutch group maps, it is important to consider a number of features. A bivalent may experience not one, but two, three, and even more chiasmata and chiasmata-related crossovers. If genes are located very close to each other, then the probability that two chiasmata will arise on the chromosome between such genes and two thread exchanges (two crossovers) will occur is negligible. If genes are located relatively far from each other, the probability of double crossing over in the chromosome region between these genes in the same pair of chromatids increases significantly. Meanwhile, the second crossover in the same pair of chromatids between the genes being studied, in fact, cancels the first crossover and eliminates the exchange of these genes between homologous chromosomes. Therefore, the number of crossover gametes decreases and it appears that these genes are located closer to each other than they actually are.
Scheme of double crossing over in one pair of chromatids between genes A and B and genes B and C. I - moment of crossing over; II - recombined chromatids AcB and aCb.
Moreover, the further the studied genes are located from each other, the more often double crossing over occurs between them and the greater the distortion of the true distance between these genes caused by double crossing over.
If the distance between the genes under study exceeds 50 morganids, then it is generally impossible to detect linkage between them by directly determining the number of crossover gametes. In them, as in genes in homologous chromosomes that are not linked to each other, during analytical crossing only 50% of gametes contain a combination of genes different from those that were present in the first generation hybrids.
Therefore, when compiling maps of linkage groups, the distances between distantly located genes are determined not by directly determining the number of crossover gametes in test crosses involving these genes, but by adding the distances between many closely spaced genes located between them.
This method of compiling maps of linkage groups makes it possible to more accurately determine the distance between relatively distant (no more than 50 morganids) located genes and identify the linkage between them if the distance is more than 50 morganids. In this case, linkage between distantly located genes was established due to the fact that they are linked to intermediately located genes, which, in turn, are linked to each other.
Thus, for genes located at opposite ends of the II and III chromosomes of Drosophila - at a distance of more than 100 morganids from each other, it was possible to establish the fact of their location in the same linkage group by identifying their linkage with intermediate genes and the linkage of these intermediate genes between yourself.
Distances between distantly located genes are determined by adding the distances between many intermediate genes, and only thanks to this they are established relatively accurately.
In organisms whose sex is controlled by sex chromosomes, crossing over occurs only in the homogametic sex and is absent in the heterogametic sex. Thus, in Drosophila, crossing over occurs only in females and is absent (more precisely, it occurs a thousand times less often) in males. In this regard, the genes of the males of this fly, located on the same chromosome, show complete linkage regardless of their distance from each other, which makes it easier to identify their location in the same linkage group, but makes it impossible to determine the distance between them.
Drosophila has 4 linkage groups. One of these groups is about 70 morganids long, and the genes included in this linkage group are clearly associated with the inheritance of sex. Therefore, it can be considered certain that the genes included in this linkage group are located on the sex X chromosome (in 1 pair of chromosomes).
The other linkage group is very small, and its length is only 3 morganids. There is no doubt that the genes included in this linkage group are located in microchromosomes (IX pair of chromosomes). But the other two linkage groups have approximately the same size (107.5 morganids and 106.2 morganids) and it is quite difficult to decide which of the pairs of autosomes (II and III pairs of chromosomes) each of these linkage groups corresponds to.
To resolve the issue of the location of linkage groups in large chromosomes, it was necessary to use a cytogenetic study of a number of chromosome rearrangements. In this way, it was possible to establish that a slightly larger linkage group (107.5 morganids) corresponds to the II pair of chromosomes, and a slightly smaller linkage group (106.2 morganids) is located in the III pair of chromosomes.
Thanks to this, it was established which chromosomes correspond to each of the linkage groups in Drosophila. But even after this, it remained unknown how gene linkage groups are located in their corresponding chromosomes. Is, for example, the right end of the first linkage group in Drosophila located near the kinetic constriction of the X chromosome or at the opposite end of this chromosome? The same applies to all other clutch groups.
The question of the extent to which the distances between genes expressed in morganids (in % crossing over) corresponded to the true physical distances between them in chromosomes also remained open.
To find out all this, it was necessary, at least for some genes, to establish not only their relative position in linkage groups, but also their physical position in the corresponding chromosomes.
This turned out to be possible only after, as a result of joint research by geneticist G. Meller and cytologist G. Paynter, it was established that under the influence of X-rays in Drosophila (like all living organisms) there is a transfer (translocation) of sections of one chromosome to another. When a certain section of one chromosome is transferred to another, all genes located in this section lose linkage with genes located in the rest of the donor chromosome and gain linkage with genes in the recipient chromosome. (Later it was found that with such chromosome rearrangements, there is not just a transfer of a section from one chromosome to another, but a mutual transfer of a section of the first chromosome to the second, and from it a section of the second chromosome is transferred to the place of the separated section in the first).
In cases where a chromosome break, when separating a region transferred to another chromosome, occurs between two genes located close to each other, the location of this break can be determined quite accurately both on the linkage group map and on the chromosome. On a linkage map, the breakpoint is located in the area between the extreme genes, of which one remains in the previous linkage group, and the other is included in the new one. On a chromosome, the location of the break is determined by cytological observations of a decrease in the size of the donor chromosome and an increase in the size of the recipient chromosome.
Translocation of sections from chromosome 2 to chromosome 4 (according to Morgan). The upper part of the figure shows the linkage groups, the middle part shows the chromosomes corresponding to these linkage groups, and the bottom shows the metaphase plates of somatic mitosis. The numbers indicate the numbers of linkage groups and chromosomes. A and B - the “lower” part of the chromosome has moved to chromosome 4; B - the “upper” part of chromosome 2 has moved to chromosome 4. Genetic maps and chromosome plates are heterozygous for translocations.
As a result of the study of a large number of different translocations carried out by many geneticists, so-called cytological maps of chromosomes were compiled. The locations of all the studied breaks are marked on the chromosomes, and thanks to this, the location of two neighboring genes to the right and left of it is established for each break.
Cytological maps of chromosomes first of all made it possible to establish which ends of the chromosomes correspond to the “right” and “left” ends of the corresponding linkage groups.
Comparison of “cytological” maps of chromosomes with “genetic” (linkage groups) provides essential material for elucidating the relationship between the distances between neighboring genes expressed in morganids and the physical distances between the same genes in chromosomes when studying these chromosomes under a microscope.
Comparison of “genetic maps” of chromosomes I, II and III of Drosophila melanogaster with “cytological maps” of these chromosomes in metaphase based on translocation data (according to Levitsky). Sp is the site of attachment of the spindle threads. The rest indicate various genes.
Somewhat later, a triple comparison of the location of genes on “genetic maps” of linkage, “cytological maps” of ordinary somatic chromosomes and “cytological maps” of giant salivary glands was performed.
In addition to Drosophila, fairly detailed “genetic maps” of linkage groups have been compiled for some other species of the genus Drosophila. It turned out that in all species studied in sufficient detail, the number of linkage groups is equal to the haploid number of chromosomes. Thus, in Drosophila, which has three pairs of chromosomes, 3 linkage groups were found, in Drosophila with five pairs of chromosomes - 5, and in Drosophila with six pairs of chromosomes - 6 linkage groups.
Among vertebrates, the best studied is the house mouse, in which 18 linkage groups have already been established, while there are 20 pairs of chromosomes. In humans, who have 23 pairs of chromosomes, 10 linkage groups are known. A chicken with 39 pairs of chromosomes has only 8 linkage groups. There is no doubt that with further genetic study of these objects, the number of identified linkage groups in them will increase and, probably, will correspond to the number of pairs of chromosomes.
Among higher plants, corn is the most genetically studied. It has 10 pairs of chromosomes and 10 fairly large linkage groups have been found. With the help of experimentally obtained translocations and some other chromosomal rearrangements, all these linkage groups are confined to strictly defined chromosomes.
In some higher plants, studied in sufficient detail, complete correspondence was also established between the number of linkage groups and the number of pairs of chromosomes. Thus, barley has 7 pairs of chromosomes and 7 linkage groups, tomato has 12 pairs of chromosomes and 12 linkage groups, snapdragon has a haploid chromosome number of 8 and 8 linkage groups have been established.
Among the lower plants, the marsupial fungus has been studied genetically in the most detail. It has a haploid chromosome number of 7 and 7 linkage groups have been established.
It is now generally accepted that the number of linkage groups in all organisms is equal to their haploid number of chromosomes, and if in many animals and plants the number of known linkage groups is less than their haploid number of chromosomes, then this only depends on the fact that they have been genetically studied insufficient and, as a result, only part of the available linkage groups have been identified.

CONCLUSION

As a result, we can quote excerpts from the works of T. Morgan:
"... Since linkage takes place, it appears that the division of the hereditary substance is to some extent limited. For example, about 400 new types of mutants are known in the fruit fly Drosophila, the features of which are only four linkage groups...
... Members of a linkage group may sometimes not be so fully linked to each other, ... some of the recessive characters of one series may be replaced by wild-type characters from another series. However, even in this case, they are still considered linked, because they remain connected together more often than such an exchange between series is observed. This exchange is called CROSS-ING-OVER - crossing over. This term means that between two corresponding series of linkage, a correct exchange of their parts can occur, in which a large number of genes are involved...
The gene theory establishes that the characteristics or properties of an individual are a function of paired elements (genes) embedded in the hereditary substance in the form of a certain number of linkage groups; it then establishes that the members of each pair of genes, when the germ cells mature, are divided in accordance with Mendel's first law and, therefore, each mature germ cell contains only one assortment of them; it also establishes that members belonging to different linkage groups are distributed independently during inheritance, in accordance with Mendel’s second law; in the same way, it establishes that sometimes there is a natural interchange - cross - between the corresponding elements of two linkage groups; finally, it establishes that the frequency of the cross provides data proving the linear arrangement of the elements in relation to each other ... "

BIBLIOGRAPHY

1. General genetics. M.: Higher School, 1985.
2. Reader on genetics. Kazan University Publishing House, 1988.
3. Petrov D. F. Genetics with the basics of selection, M.: Higher school, 1971.
4. Biology. M.: Mir, 1974.

Chromosomal theory of heredity - a theory according to which the transmission of hereditary information over a number of generations is associated with the transmission of chromosomes, in which genes are located in a certain and linear sequence. This theory was formulated at the beginning of the 20th century; the main contributions to its creation were made by the American cytologist W. Setton, the German embryologist T. Boveri and the American geneticist T. Morgan.

In 1902-1903, W. Setton and T. Boveri independently identified parallelism in the behavior of Mendelian factors of heredity (genes) and chromosomes. These observations formed the basis for the assumption that genes are located on chromosomes. Experimental evidence of the localization of genes on chromosomes was obtained later by T. Morgan and his colleagues, who worked with the fruit fly Drosophila melanogaster. Since 1911, this group has experimentally proven:

• that genes are arranged linearly on chromosomes;
• that genes located on the same chromosome are inherited linked;
• that concatenated inheritance can be disrupted due to crossing over.

The initial stage of creating the chromosome theory heredity can be considered the first descriptions of chromosomes during the division of somatic cells, made in the second half of the 19th century in the works of I.D. Chistyakov (1873), E. Strasburger (1875) and O. Büchli (1876). The term “chromosome” did not yet exist, and instead they spoke of “segments” into which the chromatin tangle breaks up, or of “chromatin elements.” The term “chromosome” was proposed later by G. Waldeyer.

In parallel with the study of somatic mitoses, there was also a study of the process of fertilization, both in the animal and plant kingdoms. The fusion of the seed nucleus with the egg nucleus was first observed in echinoderms by O. Hertwig (1876), and among plants in lilies by Strasburger (1884). It was on the basis of these observations that in 1884 they both came to the conclusion that the cell nucleus is the carrier of the hereditary properties of the organism.

The focus of attention from the nucleus as a whole to its individual chromosomes was transferred only after the work of E. van Beneden (1883), extremely important for that time, appeared. While studying the process of fertilization in the roundworm, which has a very small number of chromosomes - only 4 in somatic cells, he was able to notice that the chromosomes in the first division of a fertilized egg come half from the nucleus of the sperm and half from the nucleus of the egg. Thus:

• firstly, the fact was discovered that germ cells have half the number of chromosomes compared to somatic cells,
• and secondly, the question of chromosomes as special permanent entities in the cell was first raised.

The next stage is associated with the development of the concept of chromosome individuality. One of the first steps was to establish that somatic cells of different tissues of the same organism have the same number of chromosomes. The founder of the theory, Thomas Gent Morgan, an American geneticist and Nobel laureate, put forward hypothesis about the limitation of Mendel's laws.

In his experiments, he used the Drosophila fruit fly, which has qualities important for genetic experiments: unpretentiousness, fertility, a small number of chromosomes (four pairs), and many clearly defined alternative characteristics.

Morgan and his students found the following:

• Genes located on the same chromosome are inherited jointly or linked.
• Groups of genes located on the same chromosome form linkage groups. The number of linkage groups is equal to the haploid set of chromosomes in homogametic individuals and n+1 in heterogametic individuals.
• Exchange of sections (crossing over) can occur between homologous chromosomes; As a result of crossing over, gametes arise whose chromosomes contain new combinations of genes.
• The frequency of crossing over between homologous chromosomes depends on the distance between genes localized on the same chromosome. The greater this distance, the higher the crossing over frequency. The unit of distance between genes is taken to be 1 morganid (1% crossing over) or the percentage of occurrence of crossover individuals. If this value is 10 morganids, it can be stated that the frequency of chromosome crossings at the locations of these genes is 10% and that new genetic combinations will be identified in 10% of the offspring.

To clarify the nature of the location of genes on chromosomes and determine the frequency of crossing over between them, construct genetic maps. The map reflects the order of genes on a chromosome and the distance between genes on the same chromosome. These conclusions of Morgan and his colleagues were called the chromosomal theory of heredity. The most important consequences of this theory are modern ideas about the gene as a functional unit of heredity, its divisibility and ability to interact with other genes.

Analysis of the phenomena of linked inheritance, crossing over, comparison of genetic and cytological maps allows us to formulate the main provisions of the chromosomal theory of heredity:

• Genes are located on chromosomes.
• Genes are located on a chromosome in a linear sequence.
• Different chromosomes contain different numbers of genes. In addition, the set of genes of each of the non-homologous chromosomes is unique.
• Allelic genes occupy identical loci on homologous chromosomes.
• Genes on one chromosome form a linkage group, that is, they are inherited predominantly linked (together), due to which linked inheritance of some traits occurs. The number of linkage groups is equal to the haploid number of chromosomes of a given species (in the homogametic sex) or greater by 1 (in the heterogametic sex).
• Linkage is broken by crossing over, the frequency of which is directly proportional to the distance between genes on the chromosome (therefore, the strength of linkage is inversely related to the distance between genes).
• Each biological species is characterized by a certain set of chromosomes - a karyotype.

Topic 32. Chromosomal theory of heredity. Morgan's Law

Introduction
1. T. G. Morgan - the greatest geneticist of the 20th century.
2. Attraction and repulsion
3. Chromosomal theory of heredity
4. Mutual arrangement of genes
5. Maps of linkage groups, localization of genes in chromosomes
6. Cytological maps of chromosomes
7. Conclusion
Bibliography

1. INTRODUCTION

Mendel's third law - the rule of independent inheritance of characters - has significant limitations.
In Mendel's own experiments and in the first experiments carried out after the second discovery of Mendel's laws, genes located on different chromosomes were included in the study, and as a result, no discrepancies with Mendel's third law were found. Somewhat later, facts were found that contradict this law. The gradual accumulation and study of them led to the establishment of the fourth law of heredity, called Morgan's law (in honor of the American geneticist Thomas Gent Morgan, who first formulated and substantiated it), or the rule of linkage.
In 1911, in the article “Free segregation as opposed to attraction in Mendelian heredity,” Morgan wrote: “Instead of free segregation in the Mendelian sense, we found an “association of factors” localized close together on the chromosomes. Cytology provided the mechanism required by the experimental data.
These words briefly formulate the main provisions of the chromosomal theory of heredity developed by T. G. Morgan.

1. T. G. MORGAN - THE LARGEST GENETICIST of the 20th century.

Thomas Gent Morgan was born on September 25, 1866 in Kentucky (USA). In 1886 he was graduated from the university of this state. In 1890, T. Morgan received his Doctor of Philosophy degree, and the following year became a professor at a women's college in Pennsylvania. The main period of his life was associated with Columbia University, where from 1904 for 25 years he served as head of the department of experimental zoology. In 1928, he was invited to head a biological laboratory specially built for him at the California Institute of Technology, in a town near Los Angeles, where he worked until his death.
T. Morgan's first studies were devoted to issues of experimental embryology.
In 1902, the young American cytologist Walter Setton (1877-1916), who worked in the laboratory of E. Wilson (1856-1939), suggested that the peculiar phenomena characterizing the behavior of chromosomes during fertilization were, in all likelihood, a mechanism of Mendelian patterns . T. Morgan was well acquainted with E. Wilson himself and with the work of his laboratory, and therefore, when in 1908 he established in male phylloxera the presence of two varieties of sperm, one of which had an additional chromosome, an assumption of a connection immediately arose characteristics of sex with the introduction of appropriate chromosomes. So T. Morgan moved on to the problems of genetics. He came up with the idea that not only gender is associated with chromosomes, but, perhaps, other hereditary inclinations are localized in them.
The modest budget of the university laboratory forced T. Morgan to search for a more suitable object for experiments in the study of heredity. From mice and rats he moves on to the fruit fly Drosophila, the choice of which turned out to be extremely successful. The work of T. Morgan's school, and then most other genetic research institutions, focused on this object. Major discoveries in genetics of the 20-30s. XX century associated with Drosophila.
In 1910, T. Morgan’s first genetic work, “Sex-Limited Heredity in Drosophila,” was published, describing the white-eyed mutation. The subsequent, truly gigantic work of T. Morgan and his colleagues made it possible to link the data of cytology and genetics into a single whole and culminated in the creation of the chromosomal theory of heredity. The major works of T. Morgan “Structural basis of heredity”, “Gene theory”, “Experimental foundations of evolution” and others mark the progressive development of genetic science.
Among biologists of the twentieth century. T. Morgan stands out as a brilliant experimental geneticist and as a researcher of a wide range of issues.
In 1931, T. Morgan was elected an honorary member of the USSR Academy of Sciences, and in 1933 he was awarded the Nobel Prize.

2. ATTRACTION AND REPULSION

For the first time, a deviation from the rule of independent inheritance of characters was noticed by Bateson and Punnett in 1906 when studying the nature of inheritance of flower color and pollen shape in sweet peas. In sweet pea, purple flower color (controlled by the B gene) is dominant over red (depending on gene B), and the oblong shape of mature pollen (“long pollen”), associated with the presence of 3 pores, which is controlled by the L gene, dominates “round” pollen with 2 pores, the formation of which is controlled by the l gene.
When crossing purple sweet peas with long pollen and red sweet peas with round pollen, all first generation plants have purple flowers and long pollen.
In the second generation, among the 6,952 plants studied, 4,831 plants with purple flowers and long pollen, 390 with purple flowers and round pollen, 393 with red flowers and long pollen, and 1,338 with red flowers and round pollen were found.
This ratio corresponds well to the splitting that is expected if, during the formation of gametes of the first generation, genes B and L are found 7 times more often in the combinations in which they were found in the parental forms (BL and bl) than in new combinations (Bl and bL) (Table 1).
It seems that genes B and L, as well as b and l, are attracted to each other and can only be separated from one another with difficulty. This behavior of genes was called gene attraction. The assumption that gametes with the B and L genes in the combinations in which they were presented in the parental forms are found 7 times more often than gametes with a new combination (in this case Bl and bL) was directly confirmed in the results as called analyzing crosses.
When crossing first generation (F1) hybrids (genotype BbLl) with a recessive parent (bbll), the following split was obtained: 50 plants with purple flowers and long pollen, 7 plants with purple flowers and round pollen, 8 plants with red flowers and long pollen, and 47 plants with red flowers and round pollen, which corresponds very well to the expected ratio: 7 gametes with old gene combinations to 1 gamete with new combinations.
In those crosses where one of the parents had the BBll genotype and the other the bbLL genotype, segregation in the second generation had a completely different character. In one of these F2 crosses, there were 226 plants with purple flowers and long pollen, 95 with purple flowers and round pollen, 97 with red flowers and long pollen, and one plant with red flowers and round pollen. In this case, it appears that the B and L genes repel each other. This behavior of hereditary factors was called gene repulsion.
Since the attraction and repulsion of genes was very rare, it was considered some kind of anomaly and a kind of genetic curiosity.
Somewhat later, several more cases of attraction and repulsion were discovered in sweet peas (flower shape and leaf axil color, flower color and flower sail shape, and some other pairs of characters), but this did not change the overall assessment of the phenomenon of attraction and repulsion as an anomaly.
However, the assessment of this phenomenon changed dramatically after in 1910-1911. T. Morgan and his students discovered numerous cases of attraction and repulsion in the fruit fly Drosophila, a very favorable object for genetic research: its cultivation is cheap and can be carried out in laboratory conditions on a very wide scale, its lifespan is short and in one year you can get several dozen generations, controlled crossings are easy to implement; there are only 4 pairs of chromosomes, including a pair of sexual ones that are clearly distinguishable from each other.
Thanks to this, Morgan and his colleagues quickly discovered a large number of mutations in hereditary factors that determine traits that are clearly visible and easy to study, and were able to conduct numerous crosses to study the nature of inheritance of these traits. It turned out that many genes in the Drosophila fly are not inherited independently of each other, but are mutually attracted or repelled, and genes showing such interaction could be divided into several groups, within which all genes showed more or less strongly expressed mutual attraction or repulsion.
Based on an analysis of the results of these studies, T. G. Morgan suggested that attraction occurs between non-allelomorphic genes located on the same chromosome and persists until these genes are separated from each other as a result of chromosome breakage during reduction division , and repulsion occurs in cases where the genes being studied are located on different chromosomes of the same pair of homologous chromosomes
It follows that the attraction and repulsion of genes are different aspects of the same process, the material basis of which is the different arrangement of genes in the chromosomes. Therefore, Morgan proposed to abandon the two separate concepts of “attraction” and “repulsion” of genes and replace it with one general concept of “gene linkage,” believing that it depends on their location within one chromosome in a linear order.

3. CHROMOSOMAL THEORY OF HERITAGE

Upon further study of gene linkage, it was soon established that the number of linkage groups in Drosophila (4 groups) corresponds to the haploid number of chromosomes in this fly, and all genes studied in sufficient detail were distributed among these 4 linkage groups. Initially, the relative location of genes within a chromosome remained unknown, but later a technique was developed to determine the order of location of genes included in the same linkage group, based on the quantitative determination of the strength of linkage between them.
Quantitative determination of gene linkage strength is based on the following theoretical premises. If two genes A and B in a diploid organism are located on one chromosome, and recessive allelomorphs of these genes a and b are located on another chromosome homologous to it, then genes A and B can separate from each other and enter into new combinations with their recessive allelomorphs only in in the event that the chromosome in which they are located is broken in the area between these genes and at the site of the break a connection occurs between sections of this chromosome and its homologue.
Such breaks and new combinations of chromosome regions actually occur during the conjugation of homologous chromosomes during reduction division. But in this case, exchanges of sections usually do not occur between all 4 chromatids that make up the chromosomes of bivalents, but only between two of these 4 chromatids. Therefore, the chromosomes formed as a result of the first division of meiosis, during such exchanges, consist of two unequal chromatids - unchanged and reconstructed as a result of the exchange. In the II division of meiosis, these unequal chromatids diverge to opposite poles, and thanks to this, haploid cells resulting from reduction division (spores or gametes) receive chromosomes consisting of identical chromatids, but only half of the haploid cells receive reconstructed chromosomes, and the second half receive unchanged.
This exchange of chromosome sections is called crossing over. All other things being equal, crossing over between two genes located on the same chromosome occurs less frequently the closer they are located to each other. The frequency of crossing over between genes is proportional to the distance between them.
Determining the frequency of crossing over is usually done using so-called analytical crosses (crossing F1 hybrids with a recessive parent), although F2 obtained from selfing of F1 hybrids or crossing F1 hybrids with each other can also be used for this purpose.
We can consider this determination of the frequency of crossing over using the example of the strength of adhesion between the C and S genes in maize. The C gene determines the formation of colored endosperm (colored seeds), and its recessive allele c causes uncolored endosperm. The S gene causes the formation of smooth endosperm, and its recessive allele s determines the formation of wrinkled endosperm. Genes C and S are located on the same chromosome and are quite strongly linked to each other. In one of the experiments conducted to quantify the strength of adhesion of these genes, the following results were obtained.
A plant with colored smooth seeds, homozygous for the C and S genes and having the CCSS genotype (dominant parent), was crossed with a plant with uncolored wrinkled seeds with the CCSS genotype (recessive parent). First generation F1 hybrids were recrossed to the recessive parent (test cross). In this way, 8368 F2 seeds were obtained, in which the following splitting was found based on color and wrinkles: 4032 colored smooth seeds; 149 painted wrinkled; 152 unpainted smooth; 4035 undyed wrinkled.
If, during the formation of macro- and microspores in F1 hybrids, the C and S genes were distributed independently of each other, then in the testing cross all these four groups of seeds should be represented in equal numbers. But this is not the case, since the C and S genes are located on the same chromosome, linked to each other, and as a result, disputes with recombined chromosomes containing the Cs and cS genes are formed only in the presence of crossing over between the C and S genes, which occurs relatively rare.
The percentage of crossing over between genes C and S can be calculated using the formula:

X = a + b / n x 100%,

Where a is the number of crossing over grains of one class (grains with the Cscs genotype, derived from the combination of gametes Cs of the F1 hybrid with gametes cs of the recessive parent); c is the number of crossing-over grains of the second class (cScs); n is the total number of grains obtained as a result of analyzing crossing.
Diagram showing the inheritance of chromosomes containing linked genes in maize (according to Hutchinson). The hereditary behavior of the genes for colored (C) and colorless (c) aleurone, full (S) and wrinkled (s) endosperm, as well as the chromosomes carrying these genes when crossing two pure types with each other and when backcrossing F1 with a double recessive is indicated.
Substituting the number of grains of different classes obtained in this experiment into the formula, we obtain:

X = a + b / n x 100% = 149 + 152 / 8368 x 100% = 3.6%

The distance between genes in linkage groups is usually expressed as a percentage of crossing over, or in morganids (a morganid is a unit expressing the strength of linkage, named at the suggestion of A. S. Serebrovsky in honor of T. G. Morgan, equal to 1% crossing over). In this case, we can say that the C gene is located at a distance of 3.6 morganids from the S gene.
Now you can use this formula to determine the distance between B and L in sweet peas. Substituting the numbers obtained from analytical crossing and given above into the formula, we get:

X = a + b / n x 100% = 7 + 8 / 112 x 100% = 11.6%

In sweet peas, genes B and L are located on the same chromosome at a distance of 11.6 morganids from each other.
In the same way, T. G. Morgan and his students determined the percentage of crossing over between many genes included in the same linkage group for all four Drosophila linkage groups. It turned out that the percentage of crossing over (or the distance in morganids) between different genes that are part of the same linkage group turned out to be sharply different. Along with genes between which crossing over occurred very rarely (about 0.1%), there were also genes between which linkage was not detected at all, which indicated that some genes are located very close to each other, while others are very close to each other. far.

4. RELATIVE LOCATION OF GENES

To figure out the location of genes, it was assumed that they were arranged in a linear order on chromosomes and that the true distance between two genes was proportional to the frequency of crossing over between them. These assumptions opened up the possibility of determining the relative position of genes within linkage groups.
Suppose the distances (% crossing over) between three genes A, B and C are known and that they are 5% between genes A and B, 3% between B and C and 8% between genes A and C.
Let's assume that gene B is located to the right of gene A. In which direction from gene B should gene C be located?
If we assume that gene C is located to the left of gene B, then in this case the distance between gene A and C should be equal to the difference in the distances between genes A - B and B - C, i.e. 5% - 3% = 2%. But in reality, the distance between genes A and C is completely different and is equal to 8%. Therefore the assumption is incorrect.
If we now assume that gene C is located to the right of gene B, then in this case the distance between genes A and C should be equal to the sum of the distances between genes A - B and genes B - C, i.e. 5% + 3% = 8 %, which fully corresponds to the distance established experimentally. Therefore, this assumption is correct, and the location of genes A, B and C on the chromosome can be schematically depicted as follows: A - 5%, B - 3%, C - 8%.
Once the relative positions of the 3 genes have been established, the location of the fourth gene in relation to these three can be determined by knowing its distance from only 2 of these genes. We can assume that the distance of gene D from two genes - B and C from among the 3 genes A, B and C discussed above is known and that it is equal to 2% between genes C and D and 5% between B and D. An attempt to place gene D on the left from gene C is unsuccessful due to the obvious discrepancy between the difference in distances between genes B - C and C - D (3% - 2% = 1%) to the given distance between genes B and D (5%). And, on the contrary, placing gene D to the right of gene C gives complete correspondence between the sum of the distances between genes B - C and genes C - D (3% + 2% = 5%) to the given distance between genes B and D (5%). Once we have established the location of gene D relative to genes B and C, without additional experiments we can calculate the distance between genes A and D, since it should be equal to the sum of the distances between genes A - B and B - D (5% + 5 % = 10%).
When studying the linkage between genes included in the same linkage group, an experimental check of the distances between them, previously calculated in the same way as was done above for genes A and D, was repeatedly carried out, and in all cases a very good agreement was obtained.
If the location of 4 genes is known, say A, B, C, D, then the fifth gene can be “linked” to them if the distances between gene E and some two of these 4 genes are known, and the distances between gene E and the other two genes quadruples can be calculated as was done for genes A and D in the previous example.

5. MAPS OF LINKAGE GROUPS, LOCALIZATION OF GENES IN CHROMOSOMES

By gradually linking more and more genes to the original three or four linked genes, for which their relative positions had previously been established, maps of linkage groups were compiled.
When compiling clutch group maps, it is important to consider a number of features. A bivalent may experience not one, but two, three, and even more chiasmata and chiasmata-related crossovers. If genes are located very close to each other, then the probability that two chiasmata will arise on the chromosome between such genes and two thread exchanges (two crossovers) will occur is negligible. If genes are located relatively far from each other, the probability of double crossing over in the chromosome region between these genes in the same pair of chromatids increases significantly. Meanwhile, the second crossover in the same pair of chromatids between the genes being studied, in fact, cancels the first crossover and eliminates the exchange of these genes between homologous chromosomes. Therefore, the number of crossover gametes decreases and it appears that these genes are located closer to each other than they actually are.
Scheme of double crossing over in one pair of chromatids between genes A and B and genes B and C. I - moment of crossing over; II - recombined chromatids AcB and aCb.
Moreover, the further the studied genes are located from each other, the more often double crossing over occurs between them and the greater the distortion of the true distance between these genes caused by double crossing over.
If the distance between the genes under study exceeds 50 morganids, then it is generally impossible to detect linkage between them by directly determining the number of crossover gametes. In them, as in genes in homologous chromosomes that are not linked to each other, during analytical crossing only 50% of gametes contain a combination of genes different from those that were present in the first generation hybrids.
Therefore, when compiling maps of linkage groups, the distances between distantly located genes are determined not by directly determining the number of crossover gametes in test crosses involving these genes, but by adding the distances between many closely spaced genes located between them.
This method of compiling maps of linkage groups makes it possible to more accurately determine the distance between relatively distant (no more than 50 morganids) located genes and identify the linkage between them if the distance is more than 50 morganids. In this case, linkage between distantly located genes was established due to the fact that they are linked to intermediately located genes, which, in turn, are linked to each other.
Thus, for genes located at opposite ends of the II and III chromosomes of Drosophila - at a distance of more than 100 morganids from each other, it was possible to establish the fact of their location in the same linkage group by identifying their linkage with intermediate genes and the linkage of these intermediate genes between yourself.
Distances between distantly located genes are determined by adding the distances between many intermediate genes, and only thanks to this they are established relatively accurately.
In organisms whose sex is controlled by sex chromosomes, crossing over occurs only in the homogametic sex and is absent in the heterogametic sex. Thus, in Drosophila, crossing over occurs only in females and is absent (more precisely, it occurs a thousand times less often) in males. In this regard, the genes of the males of this fly, located on the same chromosome, show complete linkage regardless of their distance from each other, which makes it easier to identify their location in the same linkage group, but makes it impossible to determine the distance between them.
Drosophila has 4 linkage groups. One of these groups is about 70 morganids long, and the genes included in this linkage group are clearly associated with the inheritance of sex. Therefore, it can be considered certain that the genes included in this linkage group are located on the sex X chromosome (in 1 pair of chromosomes).
The other linkage group is very small, and its length is only 3 morganids. There is no doubt that the genes included in this linkage group are located in microchromosomes (IX pair of chromosomes). But the other two linkage groups have approximately the same size (107.5 morganids and 106.2 morganids) and it is quite difficult to decide which of the pairs of autosomes (II and III pairs of chromosomes) each of these linkage groups corresponds to.
To resolve the issue of the location of linkage groups in large chromosomes, it was necessary to use a cytogenetic study of a number of chromosome rearrangements. In this way, it was possible to establish that a slightly larger linkage group (107.5 morganids) corresponds to the II pair of chromosomes, and a slightly smaller linkage group (106.2 morganids) is located in the III pair of chromosomes.
Thanks to this, it was established which chromosomes correspond to each of the linkage groups in Drosophila. But even after this, it remained unknown how gene linkage groups are located in their corresponding chromosomes. Is, for example, the right end of the first linkage group in Drosophila located near the kinetic constriction of the X chromosome or at the opposite end of this chromosome? The same applies to all other clutch groups.
The question of the extent to which the distances between genes expressed in morganids (in % crossing over) corresponded to the true physical distances between them in chromosomes also remained open.
To find out all this, it was necessary, at least for some genes, to establish not only their relative position in linkage groups, but also their physical position in the corresponding chromosomes.
This turned out to be possible only after, as a result of joint research by geneticist G. Meller and cytologist G. Paynter, it was established that under the influence of X-rays in Drosophila (like all living organisms) there is a transfer (translocation) of sections of one chromosome to another. When a certain section of one chromosome is transferred to another, all genes located in this section lose linkage with genes located in the rest of the donor chromosome and gain linkage with genes in the recipient chromosome. (Later it was found that with such chromosome rearrangements, there is not just a transfer of a section from one chromosome to another, but a mutual transfer of a section of the first chromosome to the second, and from it a section of the second chromosome is transferred to the place of the separated section in the first).
In cases where a chromosome break, when separating a region transferred to another chromosome, occurs between two genes located close to each other, the location of this break can be determined quite accurately both on the linkage group map and on the chromosome. On a linkage map, the breakpoint is located in the area between the extreme genes, of which one remains in the previous linkage group, and the other is included in the new one. On a chromosome, the location of the break is determined by cytological observations of a decrease in the size of the donor chromosome and an increase in the size of the recipient chromosome.
Translocation of sections from chromosome 2 to chromosome 4 (according to Morgan). The upper part of the figure shows the linkage groups, the middle part shows the chromosomes corresponding to these linkage groups, and the bottom shows the metaphase plates of somatic mitosis. The numbers indicate the numbers of linkage groups and chromosomes. A and B - the “lower” part of the chromosome has moved to chromosome 4; B - the “upper” part of chromosome 2 has moved to chromosome 4. Genetic maps and chromosome plates are heterozygous for translocations.
As a result of the study of a large number of different translocations carried out by many geneticists, so-called cytological maps of chromosomes were compiled. The locations of all the studied breaks are marked on the chromosomes, and thanks to this, the location of two neighboring genes to the right and left of it is established for each break.
Cytological maps of chromosomes first of all made it possible to establish which ends of the chromosomes correspond to the “right” and “left” ends of the corresponding linkage groups.
Comparison of “cytological” maps of chromosomes with “genetic” (linkage groups) provides essential material for elucidating the relationship between the distances between neighboring genes expressed in morganids and the physical distances between the same genes in chromosomes when studying these chromosomes under a microscope.
Comparison of “genetic maps” of chromosomes I, II and III of Drosophila melanogaster with “cytological maps” of these chromosomes in metaphase based on translocation data (according to Levitsky). Sp is the site of attachment of the spindle threads. The rest indicate various genes.
Somewhat later, a triple comparison of the location of genes on “genetic maps” of linkage, “cytological maps” of ordinary somatic chromosomes and “cytological maps” of giant salivary glands was performed.
In addition to Drosophila, fairly detailed “genetic maps” of linkage groups have been compiled for some other species of the genus Drosophila. It turned out that in all species studied in sufficient detail, the number of linkage groups is equal to the haploid number of chromosomes. Thus, in Drosophila, which has three pairs of chromosomes, 3 linkage groups were found, in Drosophila with five pairs of chromosomes - 5, and in Drosophila with six pairs of chromosomes - 6 linkage groups.
Among vertebrates, the best studied is the house mouse, in which 18 linkage groups have already been established, while there are 20 pairs of chromosomes. In humans, who have 23 pairs of chromosomes, 10 linkage groups are known. A chicken with 39 pairs of chromosomes has only 8 linkage groups. There is no doubt that with further genetic study of these objects, the number of identified linkage groups in them will increase and, probably, will correspond to the number of pairs of chromosomes.
Among higher plants, corn is the most genetically studied. It has 10 pairs of chromosomes and 10 fairly large linkage groups have been found. With the help of experimentally obtained translocations and some other chromosomal rearrangements, all these linkage groups are confined to strictly defined chromosomes.
In some higher plants, studied in sufficient detail, complete correspondence was also established between the number of linkage groups and the number of pairs of chromosomes. Thus, barley has 7 pairs of chromosomes and 7 linkage groups, tomato has 12 pairs of chromosomes and 12 linkage groups, snapdragon has a haploid chromosome number of 8 and 8 linkage groups have been established.
Among the lower plants, the marsupial fungus has been studied genetically in the most detail. It has a haploid chromosome number of 7 and 7 linkage groups have been established.
It is now generally accepted that the number of linkage groups in all organisms is equal to their haploid number of chromosomes, and if in many animals and plants the number of known linkage groups is less than their haploid number of chromosomes, then this only depends on the fact that they have been genetically studied insufficient and, as a result, only part of the available linkage groups have been identified.

CONCLUSION

As a result, we can quote excerpts from the works of T. Morgan:
"...Since linkage takes place, it appears that the division of the hereditary substance is to some extent limited. For example, about 400 new types of mutants are known in the Drosophila fruit fly, the features of which are only four linkage groups...
...Members of a linkage group may sometimes not be so fully linked to each other, ...some of the recessive characters of one series may be replaced by wild-type characters from another series. However, even in this case, they are still considered linked, because they remain connected together more often than such an exchange between series is observed. This exchange is called CROSS-ING-OVER - crossing over. This term means that between two corresponding series of linkage, a correct exchange of their parts can occur, in which a large number of genes are involved...
The gene theory establishes that the characteristics or properties of an individual are a function of paired elements (genes) embedded in the hereditary substance in the form of a certain number of linkage groups; it then establishes that the members of each pair of genes, when the germ cells mature, are divided in accordance with Mendel's first law and, therefore, each mature germ cell contains only one assortment of them; it also establishes that members belonging to different linkage groups are distributed independently during inheritance, in accordance with Mendel’s second law; in the same way, it establishes that sometimes there is a natural interchange - cross - between the corresponding elements of two linkage groups; finally, it establishes that the frequency of the cross provides data proving the linear arrangement of the elements in relation to each other ... "

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