Polygenics and Epistasis

When the presence or absence of a gene controls the expression of another gene, it is called Epistasis.

In mice, Black fur (B) is dominant to brown fur (b). This fur color is expressed only in the presence of a dominant (C) allele. If the mouse is homozygous recessive for the c allele, no fur color will be expressed and the mouse will be white.

The presence of a dominant C gene effectively turns on the B gene, thus allowing fur color to be expressed.

No dominant C gene means that the B gene does not get turned on and no fur color will be expressed = white mouse.