Heterozygote Advantage Heterozygote advantage Zygosity Fitness Dominance Genotype Polymorphism Genetic variation Sickle-cell disease Phenotypic trait - biology - genetics 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 va... more »riability. There are several cases in which the heterozygote conveys certain advantages and some disadvantages while both versions of homozygotes are only at disadvantages. A well-established case of heterozygote advantage is that of the gene involved in sickle cell anaemia.Often, the advantages and disadvantages conveyed are rather complicated, because more than one gene may influence a given trait or morph. Major genes almost always have multiple effects (pleiotropism), which can simultaneously convey separate advantageous traits and disadvantageous traits upon the same organism. In this instance, the state of the organism's environment will provide selection, with a net effect either favoring or working in opposition to the gene, until an environmentally-determined equilibrium is reached.« less