Epistasis is the best example for gene interaction. It is a pattern of inheritance where a pair of genes situated at one locus, prevent the expression of a pair of genes situated at another locus. Such genes are called inhibiting genes or epistatic genes (Epi = above/over gene, static = standing). It is an intergenic or non allelic form of gene interaction. The basic genes, the expression of which is prevented by the epistatic genes, are called as hypostatic genes. Epistasis reduces the number of phenotypes in the F2 generation of a dihybrid cross.
A classical example of epistasis is seen in the white fowls. There are two varieties of white fowls white leg horn and white plymouth rock. A cross between a homozygous, dominant, white leghorn and a homozygous recessive white Plymouth-rock results in a progeny of F1 generation containing dihybrid white progeny and of the F2 generation consists of white and coloured fowls in the ratio of 13:3 in place of the normal phenotypic ratio of 9:3:3:1. In addition, there is a reduction in the number of phenotypes to just two.

P1 Phenotype: White Leg Horn Fowl x Plymouth Rock Fowl







CCII WHITE -- - CCIi WHITE-- --CcII WHITE-- ----CcIi WHITE
CCIi WHITE ----ccii COLOURED---- CcIi WHITE -- --CciiCOLOURED
CcII WHITE-- -- CcIiWHITE --------CcII WHITE-- ----- cCIi WHITE
CcIi WHITE ----- Ccii COLOURED -- CcIi WHITE --- cciiWHITE

WHITE FOWL: COLOURED FOWL13 : 3

Here, the basic gene C produces colour in the feathers. But the inhibiting gene I prevents the appearance of colour. Gene I interacts with gene C in such a way to suppress its expression. As a result the leg horn fowl is white CCII. The plymouth rock fowl is white since it has recessive genes ccii. In the F1 generation all the progeny are white, but heterozygous (CcIi) when these fowls are allowed to inbreed, in the F2 generation, the genotypes having gene I along with gene C will produce white fowls. The genotypes which do not have gene I give rise to coloured fowls.

Gene Interaction Examples

Phenotype------ Phenotypic ratio
1. Coat colour in mouse -- 9:3:4 Black Albino
2. Fruit colour in squash ---12: 3: 1 White: Yellow: Green
3. Flower colour in Pea ---9: 7 Purple: White
4. Fruit shape in squash--- 9:6:1 Disc:Circu!ar:Long
5. Fruit shape in shepherds purse ---15:1 Trianular: Ovoid
6. Feather colour in Fowls---- 3:3 White: Coloured