Chromosome Theory of Heredity

a theory according to which the chromosomes, which are located in the nucleus of cells, serve as carriers of genes and constitute the physical basis of heredity. That is, the continuity of the characteristics of organisms over several generations is determined by the continuity of the organisms’ chromosomes. The chromosome theory of heredity was advanced early in the 20th century after the establishment of the cell theory and of the method of hybrid analysis for the study of the hereditary properties of organisms.

In 1902 the American geneticist W. Sutton, who had observed a parallel between the behavior of chromosomes and Mendelian hereditary factors, as well as the German geneticist T. Boveri, advanced the chromosome hypothesis of heredity, according to which the Mendelian hereditary factors (later called genes) are localized in chromosomes. This hypothesis was first confirmed by studies on the genetic mechanism of sex determination in animals. These studies demonstrated that this mechanism was based on the distribution of sex chromosomes among the offspring. The chromosome theory was further substantiated by the American geneticist T. H. Morgan, who proved that the transmission of certain genes was related to the transmission of the X chromosome—that is, that sex-linked characters are inherited. For example, Morgan observed that white-eyed Drosophila females crossed to red-eyed males produced red-eyed daughters and white-eyed sons. Man has several dozen such characters, including the hereditary defects of color blindness and hemophilia. The chromosome theory of heredity was again substantiated in 1913 by the American geneticist C. Bridges, who discovered that the chromosomes of Drosophila females failed to disjoin during meiosis and that changes in the arrangement of the sex chromosomes alter the inheritance of sex-linked characters.

As the chromosome theory of heredity developed, it was discovered that the genes present in a single chromosome constitute a single linkage group that must be inherited as an aggregate. The number of linkage groups is equal to the number of chromosome pairs, which is constant for each species. The characters dependent on the linked genes are also inherited as an aggregate. Consequently, the law of the independent combination of characters was proved to have limited application. Those characters whose genes are present in different (nonhomologous) chromosomes must be inherited independently.

In the phenomenon of incomplete gene linkage, the offspring of cross-breeding have new, recombinant combinations of characters as well as the parental combinations of characters. This phenomenon, which was thoroughly studied by Morgan and his associate A. H. Sturtevant, served as the basis of the theory of the linear arrangement of genes in chromosomes. Morgan conjectured that in homologous chromosomes the linked genes found in parents in the combinations AB/AB and ab/ab may change position during meiosis in the heterozygous form → AB/ab. This results in the formation of the gametes Ab and aB together with the gametes AB and ab. Such recombinations occur because of breaks in homologous chromosomes in the region between the genes A/a B/b and the subsequent joining of the ends in a new combination:

The practicability of this process, which is called crossing-over, was demonstrated in 1933 by the German geneticist C. Stern in experiments with Drosophila, and by the American geneticists H. Creighton and B. McClintock in experiments with corn. It was proved that the farther apart the linked genes were located from each other, the greater was the likelihood of crossing-over between them. On the basis of this relationship, genetic maps of chromosomes were constructed. In the 1930’s, T. Dobzhansky proved that the arrangement of genes on genetic and cytological maps of chromosomes coincides.

According to Morgan’s theories, genes are discrete and indivisible carriers of hereditary information. In 1925 the Soviet geneticists G. A. Nadson and G. S. Filippov discovered that X rays could cause hereditary changes (mutations) in Drosophila; the same discovery was made in 1927 by the American geneticist H. Muller. This discovery, as well as the discovery that X rays could be used to accelerate the mutation process in Drosophila, led the Soviet geneticists A. S. Serebrovskii and N. P. Dubinin to formulate (1928–30) the concept that a gene can be divided into smaller units positioned in a linear series and capable of mutating. Their theory was confirmed in 1957 by the American geneticist S. Benzer in his work with the T₄ bacteriophage. The use of X rays to stimulate chromosomal aberrations enabled Dubinin and B. N. Sidorov to confirm in 1934 the gene position effect, which had been discovered in 1925 by Sturtevant—that is, the relationship between the manifestation of a gene and its position on a chromosome. The structure of a chromosome came to be regarded as both discrete and continuous.

The chromosome theory of heredity provides valuable knowledge about the universal carriers of hereditary information—the molecules of deoxyribonucleic acid (DNA) within the cell. The continuous sequence of purine and pyrimidine bases along the DNA chains has been found to include genes, intervals between genes, and markers indicating the beginnings and endings of readings within a given gene. This sequence also determines the hereditary nature of the synthesis of specific cell proteins and, consequently, the hereditary character of metabolism. DNA constitutes the physical basis of the linkage group in bacteria and in many viruses, whereas ribonucleic acid is the carrier of hereditary information in other viruses. The DNA molecules that constitute the mitochondria, the plastids, and the other cell organelles serve as the physical carriers of cytoplasmic inheritance.

By elucidating the laws of the inheritance of characters in animals and plants, the chromosome theory of heredity plays an important role in agricultural theory and practice. The chromosome theory of heredity helps specialists in selective breeding develop improved methods of introducing new animal breeds and plant varieties with predetermined characteristics. Certain aspects of the chromosome theory of heredity facilitate the development of agricultural production on a more rational basis. For example, before the devising of techniques for the artificial regulation of sex in the Asiatic silkworm, knowledge of the sex-linked inheritance of a number of characters in farm animals enabled selective breeders to discard the cocoons of the less productive sex. Before a method was developed for separating chicks according to sex by examining the cloaca, such knowledge enabled selective breeders to discard roosters. The understanding and application of polyploidy are of major importance for increasing the yield of many crops, and the study of hereditary diseases in man is based on a knowledge of the laws of chromosomal aberrations.