What is Epistasis? – Definition

  • Involves two pairs of non-allelic genes
  • Both the dominant genes affect the same character
  • One of them, when present alone or along with other dominant gene expresses itself
  • The other gene expresses itself only when it is alone.
  • The recessive forms of both the genes give rise to different phenotype
  • The phenomenon of masking of effect of one dominant gene by the other gene is called as “epistasis”, and hence the interaction is named as epistasis. The masking gene is epistatic gene and the masked one is hypostatic gene.
  • Epistasis is observed in fruit color of summer squash (white, yellow and green)

White and yellow fruit colors are monogenic dominant over green color. When plant with white fruit is crossed with that with green fruit, progeny had white fruits.

But, in F2 segregation in the ratio of 12white: 3yellow: 1green was observed. This behavior is based on ratio of dihybrid cross, so the trait must be governed by two pairs of genes.

Suppose, gene W, gene Y and gene w produce white, yellow and green colors resp. Now, the genotype of pure white fruit plant must be WWYY as yellow fruited plants are produced in F2.

The gene for yellow color compulsorily should be present there. Both the recessive genes in homozygous condition produce green phenotype. So, genotype of pure breeding green plant must be wwyy. The F2 segregation shows that the effect of gene Y is masked by gene W, when both are present together.

Cross between F1 F2
WWYY X wwyy
white X green
13 white : 3 yellow: 1 green

To know other types of gene interactions visit page Gene Interactions .

Biochemical Basis of Epistasis

For this, visit the page Biochemical Basis of Epistasis.