Question

In some plants a red pigment is synthesized from a colorless precursor by a single enzyme coded for by gene A. The red pigment can be changed into a purple pigment by another enzyme coded for by gene B. A cross between pure-breeding purple plants and white plants with the resulting F₂:

94 purple

31 red

43 white

A. Give a concise genetic hypothesis for inheritance of flower color from these results (include number of genes, number of alleles, and the relations between genes and alleles).

B. Using symbols A and B to correspond to genes A and B as described above symbolize the genotypes and their ratios for each F₂ phenotypic class.

C. If the F₁ is used in a testcross, what phenotypic ratio would you expect in the progeny?

Answer 1

The inheritance pattern is described about the recessive epistasis.

A).

Gene A consists of two alleles A(dominant) & a (recessive)

Gene B consists of two alleles B(dominant) & b (recessive)

When recessive alleles at one locus mask the expression of both (dominant and recessive) alleles at another locus, it is known as recessive epistasis.

A_B_ = Purple

A_bb or aabb = White

aaB_ = Red

B).

Purple (AABB) x White (aabb)

             AaBb (purple) ---F1

F1 x F1

	AB	Ab	aB	ab
AB	AABB (purple)	AABb(purple)	AaBB(purple)	AaBb(purple)
Ab	AABb(purple)	AAbb (white)	AaBb(purple)	Aabb (white)
aB	AaBB(purple)	AaBb(purple)	aaBB (red)	aaBb (red)
ab	AaBb(purple)	Aabb (white)	aaBb (red)	aabb (white)

Purple : Red : White = 9 : 3 : 4

C).

AaBb (F1) x aabb—Test cross

	ab
AB	AaBb (purple)
Ab	Aabb (white)
aB	aaBb(red)
ab	aabb(white)

Purple : Red : White = 1 : 1 : 2