In poultry white birds belong to two different varieties namely white leghorns or white wyandottes. Experiments reveal that the gene for white plumage of white leghorns is dominant over the gene for coloured plumage of coloured varieties. But the gene for white plumage of white wyandottes is recessive to the gene for coloured plumage of coloured varieties. Therefore the gene which produces white plumage in white leghorns is different from the gene for white plumage in white wyandottes.
A cross between a white leghorn and a white wyandotte gives an F1 of white birds with small dark flecks. When such birds are inbred, the F2 progeny segregates in the ratio of 13 white to 3 coloured birds. The experiment is explained below by postulating two genes C and I for the white leghorns:
A checkerboard for the 16 phenotypes and genotypes of the F2 birds indicates that only three out of sixteen genotypes, that is iiCC, iiCc, iiCc produce coloured birds. The white leghorns obviously contain a gene I, which in the dominant state inhibits or suppresses the expression of the dominant colour gene C, resulting in white plumage. The recessive alleles of the inhibitor gene (ii) produce coloured birds due to expression of gene C. In other words gene I is epistatic to gene C. This is a case of dominant epistasis because even one dominant allele of gene I is able to express itself.