Recessive epistasis

• Epistasis due to recessive genes is called recessive epistasis. In mice albinism (white coat) is
• produced by a recessive gene aa. 
• There is a different gene B which in the dominant state (BB and Bb) produces grey coat colour called agouti, and when recessive (bb) leads to black coat colour. 
• The recessive gene for albinism (aa) is found to be epistatic to the gene for agouti(BB and Bb), and also to its recessive, homozygous allele (bb) for black.
•  The presence of thedominant allele (AA) of the epistatic gene allows expression of gene B so that agouti (BB andBb) and black (bb) coat colours can be produced
• 
• The 9 : 3 : 4 ratio obtained is a modification of the classical 9 : 3 : 3 : 1 in which the last
  two classes (3 : 1) are phenotypically identical and are therefore added up together.
•  In human
  beings also the recessive gene for albinism shows epistasis in a similar manner.
•  Epistaticeffect is usually only in one direction, from one particular gene pair to another.