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the coexistence within a population of two or more different hereditary forms, which are in dynamic equilibrium for several or even many generations. Genetic polymorphism is most often caused either by the varying pressures and vectors (direction) of selection under different conditions (for example, at different seasons) or by the increased relative viability of heterozygotes. One type of genetic polymorphism—balanced genetic polymorphism—is characterized by a constant optimal correlation between polymorphic forms, deviation from which proves unfavorable for the species. Balanced genetic polymorphism regulates itself automatically (an optimal correlation of forms is established). The majority of genes in man and animals are in a state of balanced genetic polymorphism. Several forms of genetic polymorphism may be distinguished; analysis of these forms makes it possible to determine the effect of selection in natural populations.