Epistasis

Not to be confused with epistaxis (bleeding from the nose), epitaxis (deposition of a crystalline overlayer on a crystalline substrate), or epitasis (in classical drama, the main action of a play).

An example of epistasis is the interaction between hair colour and baldness. A gene for total baldness would be epistatic to one for blond hair or red hair. The hair-colour genes are hypostatic to the baldness gene. The baldness phenotype supersedes genes for hair colour, and so the effects are non-additive.

Example of epistasis in coat colour genetics: If no pigments can be produced the other coat colour genes have no effect on the phenotype, no matter if they are dominant or if the individual is homozygous. Here the genotype "c c" for no pigmentation is epistatic over the other genes.

Epistasis is a phenomenon in genetics in which the effect of a mutation on a trait is dependent on the presence or absence of mutations at different loci (locations on the genome), that may be on the same gene (intragenic) or on different genes, called modifier genes. In evolutionary genetics, epistasis occurs when the combined effect of two mutations is different from what you would expect by simply adding their individual effects together. This definition can be expanded to more than two loci to study higher-order epistasis, which plays an important role in shaping evolutionary trajectories.

The term epistasis was introduced in 1907 to describe the discrepancy between the prediction of segregation ratios based on the action of individual genes and the actual outcome of a dihybrid cross, and is now widely used in biochemistry, computational biology and evolutionary biology. While the nuanced meaning of the term now differs between researchers studying genetics at a functional, compositional, or at a statistical level, it still describes gene interactions at various levels.