MENDELISM

From Big Medical Encyclopedia

MENDELISM — the doctrine about discretization of the hereditary factors defining development of signs of organisms and preservation of these signs in posterity. M as the doctrine about discrete heredity developed G. Mendel and based on quantification of hybrid crossing, characterizes the first stage of formation of genetics.

M.'s emergence is historically caused by studying of interspecific crossing of plants. The first ideas of patterns of inheritance of characters of organisms arose already at the end of 18 — the beginning of 19 century. During the research of a floor at plants it is mute. botanist Kelreyter (T. G. Kolreuter), English gardener Knight (Th. A. Knight), fr. plant breeders of A. Sageret and Nodan (Ch. Naudin) showed the corpuscular nature of «elementary» ancestral features: their free recombination and not disappearance in the subsequent after hybridization generations. However inheritance of characters was studied by them totally, without their separation and quantification.

Unlike the predecessors, G. Mendel set for himself the task to consider all hybrids appearing in posterity, to distribute these forms on generations and to establish their numerical ratios. For this purpose in experiments on hybridization it used two grades of peas differing with one or several couples of mutually exclusive signs (e.g., seeds smooth or wrinkled, green or yellow, etc.).

Having assumed that to each hereditary factor there corresponds the separate sign, G. Mendel established not only independence of hereditary factors, but also quantitative ratios of the signs determined by them in all generations. Crossing the grades of garden peas differing only on coloring of seeds (yellow and green) — monohybrid crossing — G. Mendel observed in firstgeneration posterity only with yellow seeds, and in second generation when plants are provided to self-pollination, both yellow, and green seeds, the relation to - ry 3:1 equaled. The experiment made by G. Mendel on crossing of the grades of peas differing in two couples of signs — coloring (yellow and green) and a form of seeds (roundish and wrinkled), showed that in firstgeneration of posterity all seeds had yellow color and the round form. From these seeds plants were received, to-rye later self-pollination gave the seeds of the second hybrid generation differing both in a form and on coloring. Among 556 seeds of this generation G. Mendel found out 315 yellow round, 101 yellow wrinkled, 108 green round and 32 green wrinkled that he approximately corresponds to a ratio 9: 3: 3: 1.

Using alphabetic symbolics (dominant characters were designated by capital letters of the alphabet, recessive — lower case), G. Mendel showed that the numerical relations observed in experiences can be explained only at the following assumptions. First, sex cells (gametes) of hybrids bear only one hereditary element (gene); secondly, all possible types of sex cells are formed in equal quantities; thirdly, sex cells are combined with equal probability under laws of a case.

Thus, as a result of the genetic analysis which is carried out by G. Mendel it was established that each sign is indivisible and independent, and there are signs dominating and recessive (disappearing). If to keep the analysis only on one sign (monohybrid crossing), then two types of descendants are found: one with dominant, another with a recessive character (in the ratio 3:1); if to consider genetic structure of organisms, then it is possible to distinguish already three types of descendants: 1 AA (homozygous on a dominant character), 2 Aa (heterozygous), 1 aa (homozygous on a recessive character). At dihybrid crossing splitting happens on both signs, to-rye are combined in second generation independently one from another. As during the splitting on each sign there are two types of descendants concerning 3:1, at two independently heritage in second generation four types of descendants appear:

(3 + 1) X (3 + 1) = 9 + 3 + 3 + 1.

Having made experiments with trigibridny crossing and having received approximately the same results, G. Mendel drew a conclusion that it is possible to predict behavior of the hybrids differing with any quantity of signs. «If n — he claimed — number of characteristic distinctions at both initial plants, then 3 n there is a number of members of a combinational row, 4 n — number of the individuals belonging to a row, and 2 n — number of connections, to-rye remain constant. So, e.g., when initial types are various in 4 signs, a row supports 3 4 — 81 members, 4 4 — 256 individuals and 2 4 — 16 constant forms; or that the same, between 256 descendants of hybrids exists 81 various connections from which 16 konstantna».

Aiming to give qualitative biol, an explanation for quantitative patterns of inheritance (see. Mendel laws ), G. Mendel pays attention to «material structure and an arrangement of elements» in the course of formation of sex cells. However because of underdevelopment of cytology and physiology it could not describe the mechanism of a gametogenesis in details.

G. Mendel outstripped science of the time that, first, planned distinction between a genotype and a phenotype by means of introduction of the concepts «external sign» and its «deposit» which is in a sex cell; secondly, through a hypothesis of purity of gametes anticipated an essence of meiotic division; thirdly, laid the theoretical foundation for essentially new view on the nature of inheritance, i.e. opened the brightest page in the concept of the corpuscular nature of the heredity which succeeded views of the conjoint and intermediate nature of inheritance; fourthly, developed the strict, based on probabilistic and statistical type of thinking experimental method of a research of the most difficult phenomena of live.

G. Mendel's experiences could help significantly with genetic justification of theory of evolution of Darwin, against a cut in this its aspect Jenkins stated the objections in 1867. Their essence came down to the fact that as heredity is «conjoint», the sign of one of parents in succeeding generations shall have intermediate character in the beginning, then in quickly decreasing progression to go to total disappearance, and it means that it cannot become subject to natural selection. G. Mendel's researches convincingly testify to insolvency of this hypothesis. K. A. Timiryazev wrote: «... the most important result in this sense is, of course, the fact that signs do not merge..., and remain invariable, being distributed between various descendants. Jenkins's nightmare which spoiled so much blood to Darwin dissipates without trace... Thus, mendelism... eliminates the most dangerous objection which, according to Darwin, was ever made his theories» (K. A. Timiryazev. Chosen compositions, t. IV, M., 1949, p. 257, 258). However results of the researches conducted by G. Mendel were not known to Ch. Darwin, and G. Mendel's messages on them stated in his letters to Neghelli in 1866 — 1873 did not receive assessment adequate to their value.

The biology needed to pass a long way to apprehend G. Mendel's opening and to realize it biol, value. It was promoted by numerous attempts of speculative approach to interpretation of the nature of heredity by means of allocation of its elementary units. Here it is possible to mention a temporary hypothesis of a pangenesis of Ch. Darvin, according to a cut the hereditary basis of an organism is made by the gemmules which are allocated with all cells and concentrating in reproductive bodies; the statiko-dynamic concept of an equilibration of the English philosopher Spencer (N. Spencer) with its physiological units — an integral part of reproductive and somatic cells; the mechano-physiological theory it is mute. botany of Neghelli (Page W. Nageli), on a cut an organism as if is divided into a trophoplasm (plasma of somatic cells) and an idioplasm, transfer of hereditary properties a cut provide the micelles which are in sex cells; the germ plasm of A. Veysmaia located in nuclear substance of sex cells and comprising a complex a biofor with determinants to-rye is made by the hereditary inclinations distributed in chromosomes.

M.'s formation is characterized not only G. Mendel's opening though it his essence, but also and reopening of laws of Mendel by Chermak (E.Tschermak), Korrens (actually reveals To. Correns) and de Fris in 1900 that was prepared by development of biology in general. Great success was achieved in studying of a kernel and especially such morfol, educations as chromosomes; their behavior and distribution during the maturing of sex cells and fertilization was found out; specific specificity of chromosomes in the quantitative and qualitative relation is established; the doctrine about specific features and succession of a structure of chromosomes at reproduction of cells is created. On this background there is also a reopening of laws of Mendel, marking the beginning of formation of experimental genetics as special scientific discipline. However M.'s fundamental nature as genetic doctrine is realized fully much later, only as a result of comprehensive justification of the chromosomal theory of heredity (see. Morganizm ) and the synthetic theory of evolution (see. Darwinism ). Reopening of laws of Mendel not only confirmed and developed his conclusions and a method on a new experimental basis, but also put the actual and theoretical results of science received irrespective of G. Mendel's works in action. These works were involved in that specific ideological dispute, around to-rogo the theoretical thought at a boundary of 19 and 20 centuries in search of ways in the solution of a riddle of heredity and variability rotated. Due to the dominance in biology of anti-darwinist views, at many scientists Mendel's laws received the interpretation subordinated to the general problems and theoretical requirements of this direction. So, ideas of X. de Fris about mosaicity and invariances of species characters between two mutations extended to a hereditary factor (gene) was led to the fact that exaggerated still by G. Mendel and, in particular And. Idea of stability of this factor and its independence of external environment trasformirovatsya by Veysman, in particular in works of the English biologist W. Bateson and the Dutch botanist of J. Lotsy, in the thesis about its absolute invariance. W. Bateson who offered the name of new science «genetics» connected the thesis about constancy of a hereditary factor with the doctrine about mutations, to-rye were treated by it as result of «loss» of this factor. Its presence or absence was considered by W. Bateson as the decisive reason of specific specificity of live systems and that actual basis, edges defined all its anti-darwinist concept meaning, in essence, denial of evolution. Lots aggravated this false representation of neodarvinism and an early mutatsionizm, considering that evolution is possible also at constancy of types that it is connected with a recombination of invariable hereditary substance in the course of crossing.

The hyperbolization of value of gibridizatsionny processes, general for M., in evolution leading to unjustified opposition unilaterally treated Mendel's doctrines to Darwinism is connected with it. It is characteristic that it was followed by also known absolutization of those methods of experimenting, to-rye were offered by G. Mendel and his followers.

In effect, only the hybridological analysis, a comparative research of filial segregation at the level of a complete organism and indirect judgment of action of hereditary factors (genes), and also the quantification which was not followed, as a rule, by high-quality interpretation of genetic processes made metodol. M. Odnako's basis use only of these methods — at all their outstanding value — could not give the full solution of the main problems of genetics. Therefore the subsequent development of a genetic experiment was connected with searches of the new methods allowing to pass from purely quantitative research of processes of heredity and variability to qualitative.

The m was the first stage of genetics, however the stage which played a revolutionary role in biology. G. Mendel proved that hereditary factors have the discrete nature, and their transition from generation to generation is defined by variation statistical regularities.

Fundamental character of M. was emphasized by the outstanding Soviet geneticist N. I. Vavilov, to-ry wrote: «Mendel's doctrine and his further development submits one of brilliant chapters in modern biology. This doctrine remaining nearly half a century in the shadow in new conditions lit and continues to light huge area of the facts; it stimulated boundless accumulation of the actual material in biology, at the same time it led to the largest generalizations which are equally affecting both vegetable, and animal organisms including the person» (N. I. Vavilov, Izbr. works, t. 5,1965, page 338). During a certain period of development of the Soviet science, in particular after the August (1948) sessions of VASHNIL, M. was mistakenly estimated as the idealistic and metaphysical doctrine. Further development of science completely validated original positions of the doctrine of G. Mendel. Thus M. has fundamental value in the theory of selection and in genetics of the person (see. Genetics , Medical genetics , Heredity ).


Bibliography:Bogdanov E. A . Mendelism or theory of crossing, M., 1914; Bochkov N. P. Genetics of the person, page 112, M., 1978; And y with both N about in and the p A. E. Origin of genetics, M., 1967, bibliogr.; D at - and N and N. P N. General genetics, M., 1976; Korrens To. New laws of heredity, the lane with it. M, 1913; Levon-t and N of River. Genetic bases of evolution, the lane with English, page 20 and other. M. 1978; The II e N-N of e of t R. K. Mendelizm, the lane with English, M. — L., 1930; Frolov I. T. Genetics and dialectics, M., 1968; Frolov I. T. and Pastushny S.A. Mendelism and philosophical problems of modern genetics, M., 1976, bibliogr.; In a t es on W. Mendel's principles of heredity, Cambridge, 1930; FoliaMendeliana (since 1966); Ford E. B. Mendelisrn and evolution, L. — N. Y., 1960; Plate L. Vererbungslehre, Bd 1, Jena, 1932.


I. T. Frolov, S. A. Pastushny.

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