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inherited imprints (genes) located in identical parts of homologous (paired) chromosomes; they determine the developmental direction of a particular characteristic. The term was introduced by the Danish scientist W. Johannsen in 1909 to replace the term “allelomorph,” which had been proposed by the English geneticist W. Bateson in 1902. Every gene may be found in at least two allelic states (determined by its structure), one of which—the dominant allele—usually assures maximal development of a given characteristic; the other—the recessive allele—leads to partial or complete loss of the characteristic. For example, in plants, the dominant allele produces colored flowers and the recessive allele produces colorless flowers; in humans, the gene controlling eye color is present in brown-eyed persons in the form of a dominant allele (either one or a pair), and in blue-eyed persons only in the form of recessive alleles. Genes belonging to a single allelic group are denoted by a single letter—uppercase for the dominant allele and lowercase for the recessive (for example, B and b). Only one allele of a given gene can be located in each homologous chromosome. Since there are two chromosomes of every type (homologous chromosomes) in diploid organisms, there are two alleles of every type in the somatic cells of such organisms. Only one allele, together with one of the chromosomes, appears in the formation of sex cells. During fertilization (fusion of sex cells) the parity of chromosomes, and hence also of alleles, is restored. Dominant and recessive alleles may be present in the state of homozygosity (BB or bb) or heterozygosity (Bb). If a gene is present in several (more than two) different states, it forms a series of multiple alleles.
REFERENCESLobashev, M. E. Genetika, 2nd ed. Leningrad, 1967.
Müntzing, A. Genetika. Moscow, 1967. (Translated from English.)
V. S. ANDREEV