EXPRESSIVITY OF THE GENE

From Big Medical Encyclopedia

EXPRESSIVITY OF THE GENE (lat. expressus is explicit, expressive; gen.; synonym expression of a gene) — degree or a measure of phenotypical manifestation of a gene, i.e. degree and (or) the nature of expressiveness of an ancestral feature among individuals of a certain genotype, at to-rykh this sign is shown. Expressivity of a gene is closely connected with penetrance (see Penetrantnost of a gene), or manifestation, a gene (see), and also with its specificity. In total penetrance and expressivity characterize variability of phenotypical manifestation of genes.

The concept «expressivity of a gene» was entered into scientific literature by N. V. Timofeev-Resovsky and is mute. the neurologist O. Vogt, to-rye for the first time applied it in the collaboration published in 1926. Need of introduction of this concept was caused by what the term «genotype» is unambiguous and uniform defined set only of those genes, to-rye control the nek-ry ancestral features which are not changing during all individual life (see the Genotype). Antigens of erythrocytes and leukocytes of the person and animals (see Antigens), etc. belong to such signs a blood group (see Blood groups), e.g. However more often happens so that existence in a genotype of a certain gene is a necessary, but insufficient condition for full similarity of carriers of this gene on the corresponding sign. At a part of individuals — carriers of such gene (in a homozygous state for recessive genes, and in heterozygous — for dominant) it can not be shown at all (so-called incomplete penetrance), and at a part of individuals, at to-rykh this gene was shown, its expressiveness can be various, i.e. expressivity of this gene can vary (the so-called varying expressivity of a gene).

Varying E. it is well-known in medical genetics (see). So, the full syndrome of Marfan (see Marfan a syndrome) is characterized by a spider finger (see), looseness of joints, formation of aortic aneurysms and a pulmonary trunk, an incomplete dislocation or dislocation of a crystalline lens, a kyphosis (see), scoliosis (see), etc. However cases of manifestation at one patient of all a wedge, the signs characteristic of Marfan's syndrome, are rare. Cases of a «incomplete» syndrome of Marfan meet more often, and even in one family the symptom complex, as a rule is not identical at different family members.

It is necessary to distinguish manifestation of polymorphic groups of similar signs from the varying expressivity of one gene, a cut it is caused by the different genetic reasons (see the Genocopy). E.g., in medical genetics the polymorphic group of forms (not less than 7) syndromes of Elers — Danlosa who is totally characterized by different combinations, localization and expressiveness of the internal bleedings caused by ruptures of vessels, the increased distensibility of skin, looseness of joints is known. The general pathogenetic factor at all these states is disturbance of biosynthesis of collagen (see). However at different forms of a syndrome of disturbance are localized in various places of a biosynthetic chain of kollagen. Also the genetic defects causing them are various: four forms of a syndrome of Elers — Danlosa (see Desmogenez imperfect) are inherited on autosomal dominantly type, two — on autosomal recessively, and one — on the recessive type linked to X-chromosome.

The reasons varying E. there can be interindividual genotypic distinctions (genotypic millieu), variability of manifestation of genes in an ontogeny (see Ontogenesis) and influence of environmental factors. For varying E. all three reasons and interaction between them matter.

Influence of a genotypic millieu both on raised, and on lowered E. it is proved by successful artificial selection: selection of parent couples with better the expressed ancestral feature automatically accumulates in the respective line the modifiers (see the Gene) favoring to manifestation of this sign and vice versa. In some cases such modifiers are identified. About a role of a genotypic millieu in varying E. also smaller scope of intra family change of expressiveness of ancestral features in comparison with their interfamily variability testifies. Influence of variability of manifestation of genes in an ontogeny on their expressivity is illustrated by incomplete concordance (or discordance) genetically identical enzygotic (monozygotic) twins (see. Twin method) on degree and the nature of expressiveness of the same ancestral features.

Example of influence of environmental factors on E. various pigmentation of wool at animal nek-ry breeds depending on air temperature or improvement of a condition of patients with hereditary diseases serves (see) at the corresponding pathogenetic treatment (e.g., a dietotherapy, etc.).

Each of three stated reasons varying E. in any specific case can have bigger or smaller specific weight, the general rule is that E. also ontogenetic factors, and also influence of the environment on an organism as complete system in the course of ontogenesis are defined by gene interaction. Such representation about E. has great theoretical value for understanding of mechanisms of ontogenesis of live organisms and a pathogeny of hereditary diseases of the person. In medical genetics it creates a basis for searches of pathogenetic methods of correction of hereditary defects, and in selection and cultivation of page - x. plants and animals helps creation of new grades and breeds and their cultivation with conditions, optimum for the best expressiveness of economic and valuable signs.

Bibliography: Tanks N. P., Zakharov A. F. and Ivanov V. I. Meditsinskaya of the geneticist, M., 1984; Rokitsky P. F. Action field of a gene, Zhurn. ekspery. biol., it is gray. And, t. 5, century 3-4, page 182, 1929; Timofeev-Resovsky N. V. O phenotypical manifestation of a genotype, in the same place, t. 1, century 3-4, page 93, 1925; Timofeev-Resovsky N. V. and Ivanov V. I. Some questions of a developmental genetics, in book: Aktualn. vopr. sovr. geneticists, under the editorship of S. I. Alikhanyan, page 114, M., 1966; Timofeef - Ressovsky N. u. Vogt O. Uber idiosomatische Variationsgruppen und ihre Bedeutung fur die Klassifikation der Krankheiten, Naturwissenschaften, Bd 14, S. 1188, 1926.

V. I. Ivanov.

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