Each organism has only two allelic genes. However, often in nature the number of alleles can be more than two, if some locus can be in different states. In such cases, they speak of multiple alleles or multiple allelomorphism.
Multiple alleles are denoted by the same letter with different indices, for example: A, A1, A3 … Allelic genes are localized in the same regions of homologous chromosomes. Since two homologous chromosomes are always present in the karyotype, in case of multiple alleles, each organism can have only two identical or different alleles at a time. Only one of them enters the reproductive cell (together with the difference in homologous chromosomes). For multiple alleles, the characteristic effect of all alleles on the same trait. The difference between them is only in the degree of development of the sign.
The second feature is that in somatic cells or in the cells of diploid organisms contains a maximum of two of several alleles, since they are located in the same locus of the chromosome.
Another feature inherent in multiple alleles. By the nature of dominance, allelomorphic characters are arranged in a sequential order: more often a normal, unchanged character dominates the others, the second gene of the series is recessive relative to the first, however, it dominates the following, etc. One example of the manifestation of multiple alleles in humans is the blood group of the ABO system.
Multiple aleism is of great biological and practical importance, since it enhances the combinatorial variability, especially genotypic.
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