epistase

Epistase, commonly spelled epistasis in English, is a form of genetic interaction in which the phenotypic effect of an allele at one locus depends on the genotype at one or more other loci. In epistatic interactions, an epistatic gene can mask or modify the expression of alleles at a second locus (the hypostatic locus). Epistasis concerns interactions across genes rather than dominance at a single locus and can alter or obscure the expected Mendelian ratios in crosses.

Types and patterns of epistasis include several well-described forms. Recessive epistasis occurs when recessive alleles at one locus mask the expression of alleles at another locus, often yielding a 9:3:4 phenotypic ratio in dihybrid crosses (for example, coat color in some mice and Labrador retrievers). Dominant epistasis arises when a dominant allele at one locus masks the effects of the second gene, which can produce a 12:3:1 ratio in crosses. Duplicate recessive epistasis, also called complementary gene action, yields a 9:7 ratio and occurs when recessive alleles at either locus can produce the same phenotype.

A classic illustration is coat color in Labrador retrievers, where the interaction of two loci (B/b and E/e) results in yellow coats when ee, regardless of B/b genotype. The concept was introduced in the early 20th century by geneticists such as William Bateson and Reginald Punnett.

Significance of epistasis lies in its impact on trait inheritance, genetic mapping, and the interpretation of quantitative traits. Modern genetics and genomics routinely account for gene–gene interactions to understand the architecture of complex phenotypes.