For a trait conditioned by two alleles, for example, A and a, a balanced frequency between the alleles A and b is more advantageous for survival. For example, in an environmental situation in which the aa homozygous phenotype becomes incompatible with life the presence of a good number of individuals AA and Aa will result in better survival chance for the species. Another example: an environmental situation in which the dominant phenotype becomes incompatible with life; in this case the existence of heterozygous and recessive homozygous individuals in enough number may be fundamental for the survival of the species.
