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 the dominant allele (AA) of the epistatic gene allows expression of gene B so that agouti (BB and Bb) 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. Epistatic effect is usually only in one direction, from one particular gene pair to another.