It is the type of gene interaction in which one gene locus masks or modifies the phenotype of other gene locus
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.
Labrador retriever dogs.
A case of recessive epistasis is the yellow coat colour 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.
B-; ee Yellow
bb; ee Yellow
B-; E- Black
bb; E- Brown
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.