Question

A) In duplicate recessive epistasis why does having either an (aa) or (bb) genotype cause no pigment?

B) Explain the differences between Duplicate Dominant epistatis and Dominant epistatis. When doing monohybrid crosses, how do their phenotypic ratios differ in the F2 from traditional Mendelian ratios for dominant alleles?

Answer 1

A) Duplicate recessive epistasis is the phenomenon when recessive allele in either of the two locii can mask the doominant allele at the two loci. For example lets say colour of flower in seet pea is governed by two alleles A and B at two different loci.

If either recessive allele is present such as aa or bb in one loci, it will musk the effect of the dominant allele in the other locii(BB,Bb,AA, Aa) Thus in both case flower will be colourless.

Phenotypes are AAbb, Aabb, aaBB, aaBb, aabb- all will be colourless due to epistasis.

B) Dominant epistasis is the phenomenon in which a dominant allele in one of the locii musks the effect of both alleles (dominant and recessive) present in another loci.

So if colour of squash is represented by two genes W- white and G-yellow, w,g- green

Then white will always be dominant over Green (WWGG, WWGg, WwGG, WwGg, WWgg, Wwgg- all will show white)

F2 generation: WwGg X WwGg

	WG	Wg	wG	wg
WG	WWGG White	WWGg White	WwGG White	WwGg White
Wg	WWGg White	WWgg White	WwGg White	Wwgg White
wG	WwGG White	WwGg White	wwGG Yellow	wwGg Yellow
wg	WwGg White	Wwgg White	wwGg Yellow	wwgg green

Phenotypic ratio- White:Yellow: green= 12:3:1

Duplicate dominant recessive is a phenomenon where Either of the dominant genes in the two locii can musk the effect of recessive allele in both the locii. In this case lets say the yellow colour of pea pod is regulated by both W and G gene. So for presence of either of the dominant allele W or G will show white colour no matter what the 2nd allele in the other locus is.

F1 generation cross WwGg

	WG	Wg	wG	wg
WG	WWGG White	WWGg White	WwGG White	WwGg White
Wg	WWGg White	WWgg White	WwGg White	Wwgg White
wG	WwGG White	WwGg White	wwGG White	wwGg White
wg	WwGg White	Wwgg White	wwGg White	wwgg green

Phenotypic ratio- White: Green 15:1

Mendelian ratio for monohybrid cross was 9:3:3:1, whereas in case of dominant epistasis it is 12:3:1 and in case of duplicate dominant epistasis it is 15:1.