Question

For a pair of genes with alleles:

A (dominant)

a (recessive)

at the rst locus; and

B (dominant)

b (recessive)

at the second locus,

that operate in a duplicate dominant epistatic manner, what proportion of ospring from a doubly- heterozygous mating are expected to show the dominant phenotype?

A. 3/16.

B. 12/16.

C. 1 (it's dominant).

D. 15/16.

E. 0.

Answer 1

Answer - b) 12/16

Reason- Dominant epistasis is when the dominant allele of one gene masks the expression of all allelels of another gene. Let us consider, epistatic dominance in colour. We have,

Gene pair 'A' - White dominant

Gene pair 'B'- Yellow dominant to green

Here, due to epistatis, dominant white hides the effect of yellow or green.

So, the cross, AaBb × AaBb

	AB	Ab	aB	ab
AB	AABB (White)	AaBb (White)	AaBB (White)	AaBb (White)
aB	AaBb (White)	AaBb (White)	aaBB (White)	aaBb (Yellow)
Ab	AABb White)	AAbb (White)	AaBb (Yellow)	Aabb (Yellow)
ab	AaBb (White)	Aabb (White)	aaBb (Yellow)	aabb (Green)

So, phenotypic ratio = 12 (white) : 3 (yellow) : 1(green)