From Big Medical Encyclopedia

ALLELES (Greek allzlun — is mutual; synonym allels) — various forms of a condition of a gene occupying identical sites in homologous, pair chromosomes and defining a community of biochemical processes of development of this or that sign. Each gene can be at least in two allelic states determined by its structure. Existence of allelic genes caused phenotypical distinctions among individuals.

The terms «allels», «allelomorphic couple», «allelomorphism» are offered by Beytson and Saunders (W. Bateson, J. Saunders, 1902). Afterwards Yokhannsen (W. L. Johannsen, 1909) suggested to replace them with shorter — «alleles», «allelic couple», «allelism».

In initial value the term «alleles» designated only the genes defining couple of alternative Mendelian characters (see. Mendel laws ). In spite of the fact that as a matter of fact the terms «gene» and «allele» shall be synonyms, the term «allele» is used for designation of a certain kind of a gene. The concept «gene» designate locus (see) chromosomes per se irrespective of number existing And. this gene.

In each of homologous chromosomes only one allele of this gene can be located. Since diploid organisms have about two chromosomes of each type (homologous chromosomes), at cells of these organisms are present on two And. each gene. Allelic couple is formed at fertilization and can consist of identical or nonidentical A. V the first case speak about And. in homozygous, in the second — in a heterozygous state. Besides, at men's individuals of diploid organisms it can be revealed And. in a gemizigotny state. It is caused by what at the person of steam of gonosomes (XY chromosome) is not homologous. As a result of it when allelic couple cannot be made, manifestation of genes does not depend on whether they are dominant or recessive (see. Dominance ). The individual having one or several such not coupled genes, but diploid on other genes, call gemizigotny.

The name (nomenclature) of genes usually corresponds to their final effects (phenotypes), and use the English terminology. So, the recessive gene causing an achondroplasia can be called achondroplasia. For convenience of writing of genetic formulas A. designate symbols. Recessive And. it is usually designated by a lowercase first letter of the name of this gene, in particular for a gene of achondroplasia the symbol can be and. If the symbol and was already used for designation of other genes of this look earlier, then the symbol the expert or any other can be taken.

The dominant gene is designated one of the next ways: the same, but a capital letter (L), the same letter with an upper index + (and +); he is familiar + with an upper index of a symbol of a recessive allele (+ and) or is most often just familiar +. So, the genetic formula for the individual, heterozygous on a mutant recessive gene of albinism, will be with/+, for an albino with/with, and for the person with normal pigmentation +/+.

The gene which usually meets in the nature and provides normal development and viability of an organism, call a normal allele, or an allele of wild type.

The normal allele can mutate (see. Mutagenesis ). As a result of a number of consecutive mutations (see) there can be a series A. one locus. Such phenomenon received the name of a multiple allelism. Therefore to define diverse changes of any gene, it is necessary to investigate many individuals — carriers of different members of a series multiple And. People with a blood group And are subdivided into three subgroups. It is connected with existence in populations of people three various And. gene of IA — IA1, IA2 and IA3. For other allele of this IB system three different allelic forms are also known that causes allocation of three groups of people with a blood group

of V. V a crust, time is revealed by population and genetic researches more than 50 different And., controlling synthesis α-or β-polypeptide chains of a molecule of hemoglobin or enzyme of a glyukozo-6-phosphate-dehydrogenase at the person.

The main form of interaction between And. domination is (see. Dominance ). Normal (wild) And. usually dominates in relation to mutant And. Depending on the nature of interaction of normal Ampere-second mutant distinguish amorphs, hypomorphs, hyper morphs, anti-morphs and neomorphs. Amorphs — completely recessive And.; hypomorphs have the same properties, as normal And., only in the weakened degree; hyper morphs give more primary products in a cell in comparison with normal And.; anti-morphs suppress manifestation of effects normal And., aneomorfa — Ampere-second new functions, their effects not quantitatively, and qualitatively differ from effects normal And.

Though basic distinctions in operation dominant and recessive And. it is not revealed, end products of their activity (effects) are various. It is especially accurately shown in enzymes. Transformation normal dominant And. in mutant recessive often leads to synthesis of inactive enzyme. If at heterozygotes effects both are shown And., such nature of action of genes call codominant (see. Codominance ).

The only known exception of the rule of codominant action of autosomal genes is, apparently, genetic control of synthesis of polypeptide chains of immunoglobulin. The molecule of immunoglobulin consists of 2 heavy and 2 light polnpeptidny chains which synthesis is controlled by two couples of autosomal not linked genes, and in each cell only one of allelic genes of these loci is active. Such allelic exception of autosomal genes, obviously, is connected with specifics of biosynthesis of immunoglobulins.

In the history of development of the doctrine about And. the big role was played by opening of the phenomenon of a step allelism (N. P. Dubinin, A.S. Serebrovsky, etc., 1929 — 1934). In this case development of a method of interallelic complementation (see. Mutational analysis ) allowed to show that at mutations the gene can change not entirely, and through change of its separate parts. It laid the foundation for the doctrine about complex structure of a gene and considerably changed old concepts about essence And. At different changes of the same site of a gene there are gomoallel. In this case between And. no recombinations (see). At change of different places in a gene heteroalleles appear.

Pseudoalleles — closely linked loci having similar phenotypical effects. Their similarity with And. is that they are usually given together as one unit though in rare instances can recombine as a result of a crossing-over. In cis-and trans-provisions (see. Molecular genetics ) pseudoalleles cause various phenotypes. In cis-heterozygotes (ab / ++) mutant pseudoalleles show a wild or normal phenotype, and in trans-heterozygotes (and +/+) — a mutant phenotype. The group of closely linked loci is called a series of pseudoalleles, or a compound gene locus.

Gena with identical function and localization at individuals of different types call homologous. Existence of a homologous genes at individuals of different types explain them with an origin from the general parent forms. E.g., mutations of the genes controlling synthesis of enzyme of the tyrosinase participating in a chromogenesis of melanin result in inactivity of this enzyme and and as result — to emergence of albinism in different types. A homologous genes controls also synthesis VIII and IX factors of coagulant system of blood at the person and other mammals. Mutations in these genes cause development of hemophilia And yes Century.

For the majority of genes plurality of effects of manifestation therefore mutant genes cause emergence of various syndromes (see is established. Pleiotropia ). Visible effects of some genes not in all cases are shown fenotipicheskp at carriers of these genes (see. Penetrance of a gene ). Extent of manifestation of effects of allelic genes is quite often influenced by other nonallelic genes — modifiers. The last in itself have no visible effects of manifestation, but are capable to strengthen or weaken effects of the so-called key genes controlling formation of alternative Mendelian characters. Formation of a certain sign can depend also on interaction of two or more dominant nonallelic genes, each of which has no independent manifestation, and controls course of one of links of a consecutive chain of biochemical reactions. Such genes call complementary. The sign controlled by them is shown phenotypical only if all dominant And. these loci are present at an organism.

Thus, existence in population of diverse forms of the genes making allelic couples, the difficult nature of relationship in this couple influence on manifestation of this couple of nonallelic genes is the main reason for existence of phenotypical distinctions among individuals of this population on a certain sign.

The bibliography

Gershkovich of I. Genetik, the lane with English, M., 1968, bibliogr.; Dubinin N. P. General genetics, M., 1970; Lobashev M. E. Genetics. L., 1967; Medvedev N. N. Practical genetics, M., 1966, bibliogr.; Harris H. Polymorphism and protein evolution, J. med. Genet., v. 8, p. 444, 1971, bibliogr.; Wagner R. P. a. Mitсhell N. To. Genetics and metabolism, N. Y., 1964.

B. V. Konyukhov.