Heterozygote advantage is the situation where , at a gene locus , the heterozygote has greater fitness than either homozygote . A heterozygote advantage ( heterozygous advantage ) describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype . This selection favoring the heterozygote is one of the mechanisms that maintain polymorphism and help to explain some kinds of genetic variability .