Genetic models for heterosis. It is hypothesized in the diagrammed models that a phenotype or trait is controlled by multiple linked or unlinked loci (e.g., a, b, and c). (A) The dominance model. Inbred parents 1 and 2 carry slightly deleterious homozygous alleles (a and c in parent 1; b in parent 2). In the F₁ hybrid, at each locus, the superior allele A, B, or C will complement the inferior alleles a, b, or c. This complementation could cause the F₁ hybrid to exhibit a superior phenotype than the better of its parents. (B) The overdominance model. The homozygous alleles at the b locus are different between the inbred parent 1 (BB) and 2 (B′B′). When brought together in the F₁ hybrid, allele B and B′ can interact to cause a superior phenotype compared with both the parental BB and B′B′ homozygous states. (C) The pseudooverdominance model. The superior phenotype in the F₁ hybrid can be attributed to a small chromosomal region, which contains two or more different loci (e.g., a and b) that are linked in repulsion phase. The presence of superior alleles A and B in the hybrid leads to a better phenotype due to complementation, giving the impression of overdominance.