Epistasis

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.