ored leaves. You find that the purple pigment is synthesized in two steps, shown

Question

ored leaves. You find that the purple pigment is synthesized in two steps, shown in this pathway: purple pigment White green precursor Precursor The wild-type alleles of P and Q encode enzymes that catalyze each step. You also have loss of function alleles of each gene (p and q). The P gene is haploinsufficient. (a) Predict the phenotypes and proportion of each phenotypic class in the F2 progeny of a dihybrid cross. (b) After performing a dihybrid cross, you choose a purple F2 plant and backcross it to an F1, and plant a single F3 seed. What is the probability that the seed will grow into a plant with green leaves? (c) Assuming that you get a green F3 in part (c), you backcross it to a white F2. What is the probability that if you plant a single F4 seed, it will grow into a plant with purple leaves?

Explanation / Answer

It is a simple dihybrid cross but the ratio debiates from the mendals ratio:

a) In F1 the y produces 9 purple A_B_, 3 green A_bb and 4 white aa__ plants.

b) Cross of purple with F1 plant:

Purple can be A_B_

The symbol "_" indicates that this can be "A" or "a" and "B" or "b"

To become green the genotype should be A_bb

lets talk about the probability concept: 1*1/2*1*1= 1/2

1/2 because this can be "A" or smal "a" at second position.

green probability = 1/2

c) green parent:

Genotype can be A_bb

white parent genotype can be : aa__

"_" can be occupied by "B" or "b" means

aaBb or aabb

so the offspring probability to become purple:

genotype for purple: A_ B_

1*1/2*1*1/2= 1/4

purple leave probability: 1/4

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