overdominance

Overdominance, also called heterozygote advantage, is a form of natural selection in which individuals carrying two different alleles at a locus (heterozygotes) have higher fitness than individuals with either homozygous genotype. This phenomenon can maintain genetic variation in a population because both alleles persist due to the superior performance of the heterozygote.

In single-locus models, fitness is assigned to the three genotypes: w11, w12, and w22. When w12 exceeds both w11 and w22, selection favors the heterozygote. Classic real-world examples include the sickle-cell allele in malaria-endemic regions, where HbA HbS individuals have greater survival than HbAA or HbSS. Other cases involve immune genes like the MHC, where heterozygosity can enhance pathogen recognition. Overdominance can also arise from trade-offs, such as metabolic efficiency or gene dosage effects.

Overdominance promotes balancing selection, maintaining multiple alleles at a locus. The expected dynamics yield a stable equilibrium under certain fitness values, helping preserve genetic diversity over time. True overdominance is relatively uncommon and can be difficult to distinguish from pseudo-overdominance caused by linked deleterious alleles.

It is distinct from underdominance, where heterozygotes have lower fitness than either homozygote, and from simple dominance or codominance, which describe phenotypic expression rather than fitness. Overdominance remains a key concept in evolutionary biology with implications for adaptation, population genetics, and conservation.