What is epistasis?

Some of the best examples of gene interaction are those showing the phenomenon of epistasis, where the expression of one gene pair masks or modifies the effect of another gene pair. Sometimes the genes involved influence the same general phenotypic characteristic in an antagonistic manner, which leads to masking. In other cases, however, the genes involved exert their influence on one another in a complementary, or cooperative, fashion. For example, the homozygous presence of a recessive allele may prevent or override the expression of other alleles at a second locus (or several other loci). In this case, the alleles at the first locus are said to be epistatic to those at the second locus, and the alleles at the second locus are hypostatic to those at the first locus.