DOMINANT EPISTASIS-COAT COLOR IN DOGS

Due to the phenomenon of dominance a recessive allele remains obscure in the hybrid. But when two different genes which are not alleles, both affect the same character in such a way that the expression of one masks, inhibits or suppresses the expression of the other gene, it is called epistasis. The gene that suppresses is said to be epistatic, and the gene which remains obscure is hypostatic. 

DOMINANT EPISTASIS

If the dominant genotype at one locus (example II) suppresses the expression of alleles the locus B, a locus I is said to exhibit dominant epistasis over the B locus. Only if the the dominant allele is present at the I  locus can the alleles of the hypostatic B locus can be expressed. 

 In dogs whitecoat color  is dominant over the gene for coloured varieties. It develops due to the action of epiststic gene I which prevents the pigment formation by hypostatic gene B.Hypostatic gene B produces black coat color but its hypostatic allele b produces brown coat color only when I gene is recessive.The dominantb gene I does not allow  The genes B and b to function and results in white color.A cross between a white dogs of  F1  is crossed,the F2 progeny sresults in 12 white:3 Black : 1 White progenies due to epistasis.

 Reference:Fundamentals of Genetics-Rastogi