Epistasis

• Epistasis arises when the allele of one gene affects or masks the expression of another in the phenotype

An example occurs in mice where several genes determine coat colour:

• Gene A controls the distribution of a black pigment called melanin in hairs and therefore whether they are banded or not. The dominant allele A of this gene leads to hairs that have black bands while the recessive allele a produces uniform black hairs when it is present with another recessive allele a (homozygous = aa)
• Gene B controls the colour of the coat by determining or otherwise, the expression of gene A. The dominant allele leads to the production of melanin while the recessive allele b leads to no pigment and any hair will, therefore, be white when it is present with another recessive allele = bb

• The…