6 Most Important Kinds of Epistasis


The epistasis is in between two genes, that is at least a dihybrid and the phenotypes are less than 4.

(a) Dominant epistasis (12: 3: 1):

When dominant allele ‘A’ masks the expression of ‘B’ ‘A’ is epistatic gene of ‘B’. A can express itself only in the presence of ‘B’ or b allele. Therefore it is called dominant epistasis. B expresses only when ‘aa’ is present. Therefore, in 9: 3: 3: 1 ratio both 9 and 3 are expressing the ‘A’ gene the ratio is now 12: 3: 1.
Image Courtesy : upload.wikimedia.org/wikipedia/commons/2/2b/blackandchocolate.jpg


Example:
Complete dominance at both gene pairs, but one gene, when dominant, epistatic to the other.
Fruit colour in summer squash:
Gene pair A: White dominant to colour 12/16 white
Gene pair ‘B’ – Yellow dominant to green 3/16 yellow
Green – 1/16 both recessive aabb
Dominant white hides the effect of yellow or green.



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