Question

the dominant epistasis ratio is-
A. 9:3:3:1
B. 12:3:1
C. 9:3:4
D. 9:6:1

Answer 1

Hint: the dominant gene here is masking the expression of the recessive allele. We have to recall the features of dominant epistasis to answer this correctly.

Complete answer:
Let us learn a bit about epistasis. Epistasis is a genetic phenomenon where the effect of a gene mutation depends on the presence or absence of mutations in one or more other genes which are called modifier genes. Expression of one gene is affected by expression of other inherited genes. There are different types of epistasis-
Dominant epistasis- When the expression of both dominant and recessive alleles is masked by a dominant allele from another locus, it is called as dominant epistasis. Its ratio - 12:3:1.
Dominant inhibitory epistasis- when a gene suppresses the expression of another gene.
Duplicate dominant epistasis- when a dominant allele masks recessive genes at 2 locations. Ratio- 15:1
Duplicate recessive epistasis- when recessive allele masks expression of dominant gene at 2 places. Ratio- 9:7
Polymeric gene interaction epistasis
Recessive epistasis- a recessive allele masks the expression of other genes. Ratio- 9:4:3
Dominant epistasis can be explained by the example of fruit and flower colour. Eg- fruit colour in squash. Cross between heterozygous white squash gives following result-

Genotype	Fruit colour	Gene action
9 WWGG	White	Dominant white
3 WWgg	White	Dominant white
3 wwGG	Yellow	Recessive allele allows expression of yellow
1 wwgg	green	Recessive allele allows expression of green

Thus, the correct answer is- B. 12:3:1

Note: Although we observe the effects of the modifier genes in all types of epistasis, their effects are different in each type, the ratio of F2 generation of all kinds of epistasis comes out to be different. The correct ratio has to be remembered for every type.