In terms of phenotype, the impact of one genetic locus on another genetic locus.
An alternative perspective on epistasis is that multiple loci may underlie the expression of a single phenotype. The result is that changes to any one underlying locus can have a relatively large and even impact on the overall phenotype of an organism. Given epistasis, the contribution of individual loci to phenotype, in other words, can become quite complicated.
Yet another way of considering epistasis is that the of one locus has the effect of modifying the phenotypic output of another locus. Given epistasis, genes thus interact to produce specific characters, as so too can individual interact. Contrast, though, with the concept of pleiotropy.
For any one locus (locus "B"), multiple alleles may be present within a population, and these different alleles can have different epistatic impacts on the phenotype associated with another locus (locus "A"). These impacts can be to greater or to lesser extents. As measured in terms of organism phenotype, these impacts also can be, in relative terms, either positive or negative.
A detrimental mutation (locus "A") thus can be compensated for via mutations occurring in a locus that is different (locus "B") from that in which the original, detrimental mutation occurred (locus A). These compensatory mutations (in locus "B") are by definition beneficial.
The equivalent allele as found at the compensating locus (locus "B"), in this same example, could be described as detrimental. Furthermore, replacing the original mutation – the one being compensated for (i.e., as found at locus "A") ‐ with the wild type allele at this same locus (locus "A") could render the compensating mutation (at locus "B") detrimental relative the wild type allele that it replaced (also as found at locus "B").
Phenotype, in other words, is a consequence of what alleles are present within an organism's genome. Those alleles often can interact in complicated ways, and such interactions we describe, by definition, as epistatic. The contribute to fitness by a given allele in addition can be dependent on what other alleles, including at different loci, that an organism carries.
Note, though, that the concept of epistasis is distinct from the idea of , which also involves multiple loci underlying the display of individual organism characters.