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

This pattern of gene interaction given in the question typically represents recessive epistasis .

let us assume that A is for purple B for red and a,b for white,in case of recessive epistasis b will be epistatic over A, Dominant A will express in presence of B but in the presence of b, A will not express reason is b is epistatic over A. now if a cross is made AABBxaabb then the F1 would be AaBb this would be purple reason is B is present.

when self is made i.e AaBbxAaBb then we get three different phenotypes one is Purple all having AB this will be 9.

AAbb or Aabb this genotype will be white reason is there is b with A so b shows its expression so this is 3

aabb will be one which is also phenotypically white so the total white are 4/16

next is with genotype aaBB or aaBb this will give red flowers , as dominant A is absent the pigment for purple is not made so it will be red this will be 3/16 .

So 9/16 is purple

3/16 is red

4/16 is white . as per the question also this ratio approximately fits , which is a case of recessive epistasis.

X---------B enzyme-----> Red pigment------Aenzyme ------>purple pigment.

C. If a test cross is made then AaBbxaabb then we have four gametes from AB,Ab,aB,ab which will fuse with ab

which will result in

AaBb - purple

Aabb - white

aaBb - red

aabb - white so purple 1/4, white 2/4,red 1/4 so ratio of purple to red to white is 1:1:2 respectively.

hope this helps.