In the presence of a B allele, black pigment can be made from a colorless substance. In the presence of an A allele, the black pigment is deposited during the development of hair in a pattern producing the agouti phenotype.

If the aa genotype occurs, all of the hair remains black.

If the bb genotype occurs, no black pigment is produced, regardless of the presence of the A or a alleles, and the mouse is albino.

Therefore, the homonzygous b genotype masks or suppresses the expression of the A allele; this is referred to as recessive epistasis.