From Big Medical Encyclopedia

MENDEL LAWS ( G. J. Mendel , 1822 — 1884) — rules of inheritance. the establishing numerical ratios, in to-rykh are shown separate ancestral features and their combinations in hybrid posterity at a syngenesis. At diploid bisexual organisms (separate and bisexual) set of ancestral features (an integral phenotype) is defined by set of a large number of the genes localized in a pair (diploid) set of chromosomes, to-ry is formed in the course of fertilization from merge of the unary (haploid) sets which are contained in female and men's sex cells (gametes). Manifestation of separate ancestral features is controlled by a combination alleles (see) corresponding chromosomal genes. The individual who received identical alleles of any gene from both parent gametes is called a homozygote on this gene and AA or aa is designated, e.g.; the individual who received different alleles — a heterozygote is also designated by Aa (the individual bearing one or several not coupled genes, but diploid on other genes, is called gemizigoty). In the course of a gametogenesis at a reduction of chromosomal complement in meiosis (see) homozygotes form the same gametes, i.e. homozygotes of a genotype of A A give only gametes And, and homozygotes aa — only gametes and. Meiosis at heterozygotes forms equal number of gametes of two types, e.g., of a heterozygote of Aa — equally gametes And N and. During the crossing between the same homozygotes (e.g., AA X AA or aa X aa) all posterity has a genotype of the parents (AA or aa, respectively). Numerical ratios of different genotypes and the phenotypes (ancestral features) corresponding to them in posterity from crossing different gomo-and heterozygotes (e.g., AA X aa, Aa of X Aa, etc.) M. z is defined. These laws describe as results of crossing of the forms differing with alleles of one gene (monohybrid crossing) and the forms differing with alleles more than one gene. If at the same time initial forms differ on two genes controlling different signs, crossing is called dihybrid (e.g., AA BB x aa bb), on three — trigibridny (e.g., AA BB CC x aa bb cc), and generally — polyhybrid.

Fig. 1. The scheme illustrating the first law of Mendel (uniformity of hybrids of firstgeneration): And yes and — alleles of any autosomal gene; ♀ — a female individual, ♂ — a male individual.
Fig. 2. The scheme illustrating disturbance of uniformity of hybrids of firstgeneration but to the signs inherited it is linked to a floor: at hybrids of firstgeneration of HAHa = HaHA, but XAY ≠ XaY; X and Y — gonosomes; And yes and — alleles of any gene of X-chromosome; ♀ — a female individual; ♂ — a male individual.

G. Mendel and his early followers on the basis of results of experiments formulated three rules of inheritance. The first law (rule) of Mendel (the law of uniformity, the law of domination) says that the posterity of firstgeneration from monohybrid crossing between homozygotes is uniform irrespective of direction of cross (?AA x of a ?a = a ?aa x ?AA), and its phenotype is defined by the relations of dominance (domination) between the alleles participating in crossing (♀ — a symbol of a female individual, ♂ — a symbol of a male individual). E.g., in Mendel's experiments on crossing between homozygous purple and floral and white-flowered kinds of peas the purpurnotsvetkovost completely dominated and all hybrid posterity of firstgeneration was purple and floral. The scheme of monohybrid crossing illustrating the first law of Mendel is provided on fig. 1. This law is fair for inheritance of characters, controlled by autosomal genes, and for the signs controlled by genes of gonosomes it partially is broken. At the person and many animals pair sets of chromosomes at individuals of a different floor are various: female set contains two X-chromosomes, and in men's — one H-and one Y-chromosome, edge practically does not contain genes, homologous to genes of X-chromosome. Therefore cells of a male body contain unary (half from women's) gene pattern of X-chromosome, they are gemizigotna (poluzigotna) on these genes. This circumstance does results of different directions of cross by inadequate (fig. 2).

Fig. 3. The scheme illustrating splitting on gomo-and heterozygotes in generations of hybrids at monohybrid crossing: And yes and — alleles of any autosomal gene.

The second law (rule) of Mendel (the law of splitting) says that during the crossing between monohybrids of firstgeneration (Aa of X Aa) in posterity of second generation splitting on gomo-and heterozygotes on average a ratio 1/4AA is observed: 2/4 Aa: 1/4aa, at the same time the ratio of phenotypes is defined by the relations of domination between the alleles traced in crossing. E.g., at complete dominance of an allele And over an allele and when homozygotes of AA phenotypical do not differ from heterozygotes of Aa, the «classical» ratio — 3/4 individuals with a dominant character and 1/4 individuals — with recessive is observed. In further generations inbreeding (see) homozygotes remain constants (AA x AA >—AA and aa x aa — >aa), and heterozygotes of Aa give the same splitting again, as well as heterozygotes of firstgeneration (fig. 3).

Fig. 4. The table for definition of genotypes of individuals of second generation and numerical ratios between them («Pannet's lattice»): And yes and, In and b — alleles of two not linked autosomal genes; ♀ — a female individual; ♂ — a male individual. Shading showed splitting on a phenotype of F2 concerning 9: 3: 3: 1 at complete dominance of an allele And over and and In over b for lack of interaction between these genes. In an extreme left column of the table write down all possible types of female gametes, and in an upper row — men's, and their combinations (genotypes of zygotes) have on crossings of the corresponding columns and ranks.

The third law (rule) of Mendel (the law of a free combination) claims that in second generation of polyhybrid crossings splitting on each factor pair happens irrespective of splitting on other couples. For finding of genotypes of individuals of second generation and numerical ratios between them usually use the table called by Pannet's (fig. 4) lattice. The law of a free combination is observed when all genes traced in crossing are located in different couples of chromosomes. Otherwise the frequency of a recombination is determined not by Mendel, and depends on distance between these genes on the genetic map of the corresponding chromosome (see. Recombination , Chromosome map ).

There are also nek-ry general conditions for M.'s manifestation z. First, M. z. are fair only for chromosomal genes and are unfair for hereditary cytoplasma factors (see. cytoplasmic inheritance ). Secondly, M. z. are statistical and are carried out rather precisely only on material, big on number (about hundreds and thousands of individuals). It is caused by accident of distribution of different couples of chromosomes in meiosis and equal probability of a meeting of different types of gametes at fertilization. Thirdly, the numerical ratios of phenotypes predicted by M. z., are observed only when all traceable alleles have full penetrance (see. Penetrance of a gene ), and between different genes in polyhybrid crossings there is no interaction (e.g., a complementarity or an epistasis).

Basis for M.'s formulation z. long-term (1856 — 1863) experiments on crossing of several kinds of peas served. G. Mendel's work was not noticed by his contemporaries, and only in 1900 the results received by it were independently reproduced by Korrens (To. Correns) in Germany in experiences on peas and corn, H. Fris in Holland in experiences on representatives of several childbirth of plants and E. Tschermak in Austria in experiences on peas. Soon similar results were received by W. Bateson in England in experiences on hens and L. Cuenot in France in experiences on mice. Only then obshchebiol. the importance of the patterns of transfer of ancestral features and properties opened by G. Mendel was realized by biologists. The term «Mendelian character» was entered, the Crimea designated such ancestral feature, to-ry in posterity is inherited, according to M. z., but to monogenic type. This sign shall be obligatory discrete, but not continuous. In the latter case it is not about mendeliruyushchy, and about polygenic signs.

Modern sounding of M. z. received with completion of creation by T. Morgan and his school of the chromosomal theory of heredity. At the beginning of 20 century it was revealed that M. z. are applicable also to separate ancestral features of the person, and modern catalogs of mendeliruyushchy heredity of the person include apprx. 2 thousand signs. M z. under the conditions of their use formulated above completely keep the value. They are an obligatory basis of all types of the genetic analysis of diploid organisms, including the genealogical analysis at the person. On their basis selection programs in crop production and livestock production are under construction, they form a basis and during the forecasting of a genotype and phenotype of posterity in medicogenetic consultation.

See also Genetic analysis .

Bibliography: N. P. Genetik's tanks of the person, M., 1978; Gershenzon G. M. Fundamentals of modern genetics, Kiev, 1979; JI about and sh e in M. E. Genetik, D., 1967; Makkyyusik V. A. Ancestral features of the person, a per with English, M., 1976; Mendelg. Experiences over vegetable hybrids, M., 1965.

V. I. Ivanov.