Index >> Genetic Interaction >> Kinds of Epistatic Interaction

Example
Dominant epistasis in dogs : Among dogs, the colours of coats depend upon the action of two genes. One gene locus· has a dominant epistatic inhibitor allele (I) of coat colour pigment. The allele I prevents the expression of colour allele at another independently assorting, hypostatic gene locus (8 or b) and produces white coat colour. The alleles of hypostatic gene locus (BB, Db, or bb) express only when two recessive alleles (ii) occur on the epistatic locus, i.e., ii BB or ii Bb produces black-and ii bb produces brown. When, two such white coat coloured dogs are crossed in F1 the white, black and brown coat colours appear in 12 : 3 : 1 ratio as shown in the following figure :

The agouti coat colour is controlled by a gene A which is hypostatic to recessive allele c. Thus, cc AA, cc Aa and cc aa genotypes produce albino phenotypes. Further the dominant allele C in the absence of A gives coloured (black) mice, i.e., CC aa and Cc aa genotypes give black coat colours. Moreover, in the presence of dominant epistatic allele C, the hypostatic dominant allele A gives rise to agouti coat. Thus, CCAA, CcAA, CcAa and CC Aa genotypes produce agouti coat. When black mice (CCaa) are crossed with albino (ccAA), agouti mice (CcAa) appears in F₁. In F1 9 agouti, 3 black and 4 albino individuals are obtained due to recessive epistasis as shown in the following figure: