The production of purple pigment in a flower requires two enzymes in a pathway,

Question

The production of purple pigment in a flower requires two enzymes in a pathway, which are encoded by genes G and H. These genes have dominant alleles that encode functional enzymes, and recessive alleles that encode non-functional enzymes. Assume the colorless intermediate made by protein G is the substrate for protein H. G and H are on different chromosomes, and a GGhh is crossed with a ggHH to produce a double heterozygous F1. What distribution of phenotypes is expected from a testcross of the F1 with a gghh?

a. 25% purple, 75% white

b. 75% purple, 25% white

c. Impossible to determine

d. 100% white

e. 50% purple, 50% white

Explanation / Answer

B. 25% purple, 75% white is the correct answer

The parents have the following genotypes:

GGhh              ggHH

A cross between these two parents results in the following F1 progeny:

gH

gH

Gh

Gg Hh

Gg Hh

Gh

Gg Hh

Gg Hh

All of the F1 progeny are double heterozygous. When they are test crossed with a gghh genotype, the following genotypes of progeny are expected:

Since homozygous recessive genotypes encode nonfunctional enzymes, 75% are colorless, where the remaining 25% produce purple flowers. Hence, the F2 progeny will have 25% purple and 75% white flowers.

Therefore, option B. 25% purple, 75% white is correct

gH

gH

Gh

Gg Hh

Gg Hh

Gh

Gg Hh

Gg Hh

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