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Modern Genetic Analysis Chapter 3. Gene Function
Solved Problem

2.

Yeast (a haploid) can normally make its own proline. Six auxotrophic mutants were obtained that could not make proline and needed proline to be supplied in the growth medium. Two other compounds structurally related to proline (GSA and glutamine) were added to the mutants to determine if they could cause growth instead of proline. The results were as follows, where "yes" means the compound permitted growth and "no" means it did not:

a. How many different responses are there among the six mutants?
b. What do the different responses represent?
c. What is the relationship between the three compounds proline, GSA, and glutamine?
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Solution
a. There are three different groups:

Group A (mutants 2 and 4) can use proline only.

Group B (mutants 1, 5, and 6) can use proline or GSA.

Group C (mutant 3) can use proline, or GSA, or glutamine.

b. These probably represent mutations in different genes.
c. The different genes code for three enzymes (enzymes A, B and C, corresponding to genes A, B, and C) in a biochemical pathway in which the end point is proline. The pathway must be as follows, because only this arrangement explains the pattern of restoration of growth by supplementation after a block in the pathway.
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