From Big Medical Encyclopedia

DOMINANCE (Latin dominans, dominant[is] dominating; a synonym domination) — manifestation of a sign only one of parents and more or less full suppression of an alternative sign of other parent at the hybrids received by crossing of two, the individuals hereditarily differing. Operating with terms of genetics, one may say, that dominance is a dominance in a phenotype of an individual (see. Genotype ), heterozygous on any gene, one allele over another. Prevailing allele (see), and also a sign which it defines call dominant, and the allele (sign) masked by it — recessive. Does not mean absence at a hybrid of the hereditary factors necessary for development of the suppressed (recessive) sign.

The hybrids having dominant characters nevertheless can transfer to a part of the posterity the recessive characters hidden at them. Such D. it is especially characteristic of the ancestral features which are shown on morfol, level.

The phenomena of dominance and recession are characteristic of all diploid organisms, including and of the person, animals and plants. The relations of codominance (see) are more characteristic of the signs registered at the level of primary products of genes (proteins) when in a phenotype each of two alleles any is shown locus (see). E.g., in system of blood groups of AB0 the combination in a genotype of the individual of alleles And yes In gives a phenotype of AV (see. Blood groups ).

Usually dominant and recessive alleles of one gene designate identical symbols, but write a symbol of a dominant allele from a capital letter, and recessive — with lower case. E.g., at the person normal pigmentation of a body is defined by a dominant allele And, and its absence (albinism) a recessive allele of the same gene and. Therefore, all persons who inherited at least one allele And, i.e. having a genotype of AA or A and, will be normally pigmented, and the persons who inherited an allele from both parents and, i.e. having a genotype aa, will be full albinos (see. Albinism ). It is clear, that all children from marriage between two full albinos will also be full albinos, and in marriages between normally pigmented faces of a genotype of Aa a part of children — 1: 4 (see. Mendel laws ) — will inherit the full albinism which is not shown at their parents.

Should be considered not as some stable property of one certain allele from a factor pair of this gene and as result of action of an allele in the general «system of a genotype».

In some cases the relations of dominance — recession between two alleles of a gene are constants and are not broken at change of conditions of an external and genotypic millieu, i.e. in whatever combinations to alleles of other genes and in what physiologically acceptable conditions of the environment such factor pair in a heterozygous state met, always among these two alleles dominant will be the same allele. Along with it a number of genes is known (at different organisms), the relations between which alleles are changeable also under the influence of other nonallelic genes, so-called modifiers, can change on opposite. Besides, within the same genotype the relation of dominance — recessions can influence restructurings of chromosomes. Transfer of a dominant allele in an environment, new to it, can lead to change of extent of manifestation of. This effect which received the name of a position effect of a gene (see. Gene ), in experiences on a drosophila found A. H. Sturtevant, H. P. Dubinin and B. N. Sidorov. The return transfer of the site of a chromosome to the initial position leads to D. Eto's recovery shows that the reason of decrease or D.'s loss really is the position effect, but not «damage» of an initial dominant allele in the course of reorganization.

Extent of manifestation of a dominant allele depends on external conditions, and also on action of other genes (modifiers). Full D. and full recession exist as extreme cases. Between them there can be all transitions from strong D. through weak to intermediate inheritance and to weak and strong recession. Distinguish the following main forms D.:

1. Full D. — a heterozygous form A and on a phenotype corresponds to AA.

2. Semi-dominance (partial, incomplete D.) — a heterozygous form of Aa on a phenotype is intermediate between AA and dd.

3. Unstable, labile D. — manifestation of the corresponding alleles (dominant or recessive) in a heterozygous state depends on a genotypic millieu and external conditions.

4. Alternating D. — in the course of ontogenesis is shown one, and then other allele from a heterozygous factor pair in the beginning.

5. Pseudo-domination — domination of the recessive gene which is present not at a form of a factor pair, and at singular (a gemizigotny condition of a gene).

Representation is widespread that from two alleles of one gene («normal» or «pathological») the «normal» allele and that the majority of again arising genovariations retsessivno in most cases dominates. The most widespread theory of evolution is based on this representation. However results of «inventory» of ancestral features of the person show that idea of D. of the majority of «normal» alleles cannot be considered universal: from the 20 century of two thousand, known to the second half hereditary diseases (see) the person about a half is caused by dominant mutations.

Bibliography Dubinin N. P. General genetics, M., 1976; Lobashev M. E. Genetics, L., 1967, bibliogr.; Makkyyusik V. A. Ancestral features of the person, the lane with English, M., 1976; Stern K. Fundamentals of genetics of the person, the lane with English, M., 1965.

V. I. Ivanov.