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the state, inherent in every hybrid organism, in which homologous chromosomes carry different forms (alleles) of a given gene or differ in the arrangement of genes (structural heterozygosity).

The term “heterozygosity” was first introduced by the English geneticist W. Bateson in 1902. Heterozygosity is the result of the union of gametes of different genetic or structural makeup. Structural heterozygosity arises with the chromosomal rearrangement of one of the homologous chromosomes; it may be detected in meiosis or in mitosis. Heterozygosity is revealed by crossing a hybrid with its homozygous-recessive parental form. Heterozygosity, as a rule, is a consequence of the sexual process, but it can arise as a result of mutation (for example, in the homozygote AA, where one of the alleles has mutated: A→A’). The effect of harmful or lethal recessive alleles is suppressed in a heterozygote by the presence of the corresponding dominant allele and becomes apparent only with the transfer of the allele to a homozygous situation. Heterozygosity is widespread in natural populations and is apparently one of the reasons for hybrid vigor (heterosis). The masking action of the dominant alleles in heterozygosity allows the preservation and diffusion through a population of harmful recessive alleles, which should be unmasked in the course of breeding and selection as well as in making medical and genetic prognoses (for example, through evaluative testing of a stock by studying its progeny).


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