A form of dominance in which the expression of the heterozygote is outside the range defined by the expressions of the homozygous genotypes and most closely resembles the expression of the homozygous dominant genotype.
The expression of two alleles in a heterozygous condition of a phenotypic value for some characteristic that lies outside the range of the two corresponding homozygotes; a possible basis for heterosis or heterozygote advantage.
Overdominance is an alternate term for heterozygote advantage, a condition in genetics where the phenotype of the heterozygote is fitter than the phenotype of either homozygote. The concept of overdominance has been used by plant breeders crossing inbred strains and selecting for desired characteristics. The theory that heterosis or hybrid vigor could be explained by heterozygote advantage became known as the overdominance hypothesis.