In trying to ﬁnd evidence of gene interaction, one approach is to look for cases of a type of gene interaction called epistasis. This word means “stand upon,” referring to the ability of a mutation at one locus to override a mutation at another in a double mutant.The overriding mutation is called epistatic, whereas the overridden one is hypostatic. Epistasis results from genes being in the same cellular pathway. In the case of a simple biochemical pathway, the epistatic mutation is of a gene that is farther “upstream” (earlier in the pathway) than that of the hypostatic. The mutant phenotype of the upstream gene takes precedence, no matter what is going on later in the pathway.
A case of recessive epistasis is the yellow coat color of some Labrador retriever dogs.
Two alleles, B (black) and b (brown), coats, respectively. The two alleles produce black and brown melanin.
The allele e of another gene is epistatic on these alleles, giving a yellow coat.
Therefore the genotypes B/– ; e/e and b/b ; e/e both produce a yellow phenotype,
whereas B/– ; E/– and b/b ; E/– are black and brown, respectively.
This case of epistasis is not caused by an upstream block in a pathway leading to dark pigment. Yellow dogs can make black or brown pigment, as can be seen in their noses and lips. The action of the allele e is to prevent deposition of the pigment in hairs. In this case, the epistatic gene is developmentally downstream; it represents a kind of developmental target that has to be of E genotype before pigment can be deposited.