From Big Medical Encyclopedia

The CHROMOSOMAL THEORY of HEREDITY (Greek chroma color, coloring - f-soma a body) — the main theory of modern genetics, according to a cut the main material carriers of heredity are chromosomes and genes located on them in a certain linear sequence.

Bases of the theory are formulated and experimentally confirmed by T. Morgan and his employees A. Sturtevant, H. J. Muller and Bridzhiz (S. V. to Bridges) at the beginning of 20 century. Laws of heredity and variability are determined by X. so-called behavior of chromosomes in a mitosis (see), meiosis (see) and at formation of a zygote (see Mendel laws).

In 1865 Mendel, studying numerical ratios of qualitative characters in the hybrid posterity received from crossing of the plants of peas differing from each other suggested about existence of hereditary factors (after called genes) and purity of sex cells — gametes (see Gametes, the Gene). According to this hypothesis, manifestation of each ancestral feature at organisms with a syngenesis is controlled by couple of hereditary factors or on modern terminology a factor pair (see Alleles) one gene, one of to-rykh is given to a germ by an ovum, and another — spermiy. In the course of growth and development all factor pairs of various genes are transferred from a cell to a cell, being reproduced (see the Reproduction of chromosomes) in each cellular cycle, and cause manifestation of the corresponding ancestral features. During the maturing of sex cells all factor pairs are distributed in such a way that mature gametes contain only on one allele for each ancestral feature, i.e. are «pure» (not hybrid). Distribution of members of each factor pair between the ripening sex cells happens irrespective of distribution of members of other couples. In the course of fertilization men's and female gametes merge, and their unary sets combine, forming a pair set of new generation. This hypothesis of G. Mendel anticipated opening of chromosomes, mechanisms of cell fission and cytologic bases of fertilization. In the last quarter of 19 century the beginning of 20 century E. Strasburger, Bowe-ri (Th. Boveri) and Wilson (E. V. of Wilson) and other scientists opened existence of chromosomes (see) and proved that to everyone biol. to a look svoystven a certain, constant chromosomal complement (see). It was revealed that paired relationship of set is recovered in the course of fertilization, chromosomes of different couples are nonidentical, individual and implementation of normal ontogenesis requires full chromosomal complement. Afterwards mechanisms of behavior of chromosomes in a mitosis and meiosis were studied. W. Sutton in 1902 generalized data on a structure and functioning of chromosomes and pointed to full parallelism of chromosomal cycles with behavior of Mendelian hereditary factors.

Always to a large number of ancestral features, to-rye, according to Mendel, X shall recombine discrepancy of usually small chromosome number independently (see the Recombination). de Fris explained with the fact that each of chromosomes contains a large number of hereditary factors, and in meiosis homologous (structurally identical) chromosomes freely exchange alleles, it also provides an independent combination of the different factor pairs located in the same couple of homologous chromosomes. W. Bateson, Saunders (E. V. Saunders) and Pannet (R. Pages of Punnet) showed that the law of an independent combination is not universal: nek-ry

couples of ancestral features recombine less often than expected and remain preferential in those combinations at what they were present at initial parent forms. This phenomenon was called by them coupling of signs (and the corresponding hereditary factors, genes). At the same time coupling of nonallelic genes does not happen absolute, and the force of adhesion of one couple genes is rather constant and does not depend on at what of possible combinations these genes were present at initial parent forms. Justification of the chromosomal theory of heredity was opening of chromosomal mechanisms of sex determination (see Paule, Chromosomes).

Decisive proofs of X. so-called were received by T. Morgan and his employees during the studying nasledo a vaniye of signs at a fruit fly of a drosophila (see) when it was shown that set of ancestral features of a drosophila breaks up to not blocked groups of heritage (linkage groups), and within group all signs are inherited is linked - but, and any sign of one group independently recombines another with any sign. Total number of linkage groups — four — was equal to chromosome number in haploid set. Inheritance of characters, belonging to three of four group-and coupling at a drosophila, occurred irrespective of a floor. Signs of the fourth group were inherited is linked to a floor. Accessory of the genes inherited it is linked to a floor, to X-chromosome it was proved by Bridzhiz in direct experiments and at the same time the new phenomenon — not - the discrepancy of chromosomes conducting to an aneuploidy was opened for them (see Chromosomal complement). At the person the aneuploidy is an etiological basis of chromosomal diseases (see).

Important experimental confirmation of X. establishment of an arrangement of genes on chromosomes — creation of genetic maps of chromosomes was so-called (see. Chromosome map). The parallel genetic and cytologic analysis of hybrid posterity showed that the recombination of the studied linked external ancestral features steadily is followed by a recombination of the corresponding marker chromosomes.

T. Morgan and his employees suggested that the frequency of a recombination of linked genes is proportional to distance between them on a chromosome. In series of crossings they determined the frequency of a recombination between all nonallelic genes known for it in all four linkage groups at a drosophila. As a result genes of each linkage group were built in the unique uneven linear row which received the name of the genetic map of chromosomes. Conclusions were drawn that genes on chromosomes are located in the constant sequence in quite certain points (loci) and that exchange between genes does not affect their integrity. Later restructurings of chromosomes were open (see the Mutation), as a result to-rykh the whole blocks of chromosomal material can move as within one chromosome — inversion (see), transpositions, and between chromosomes — translocations (see) that leads respectively to change of localization of genes.

Establishment of full parallelism on genetic and cytologic maps of chromosomes served in the sequence of genes as final justification of X. so-called. Now this parallelism is found not only in a drosophila, but also in all genetically studied species of plants, microorganisms and animals, including and in the person. Discovery of cytoplasmic inheritance does not contradict the chromosomal theory since on this mechanism less than 1% of all signs are inherited (see cytoplasmic inheritance). X. so-called explains all known patterns of gene interaction. The chromosomal theory of heredity serves not only for theoretical justification of mechanisms of heredity and variability, but also has great practical value for exact establishment etiol. factors of genetically caused pathology at the person.

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