Exam III is graded and the scores have been uploaded to WebCT. The mean was 68.6, range was 20-97. Quite a few people were hit hard by this exam, and in particular, question 4 on page 2 was one of those problems where you either got it and did well, or missed the key insight and spent an hour pounding your head against the desk trying to figure it out.

Here's the problem:

4 (20 pts). You have mapped a set of markers on chromosome 2 of the fruit fly such that the wild type chromosome can be diagrammed like so: A—o—B—C—D—E The (o) marks the location of the centromere, and assume that all of these genes are close together (< 5mu) except A, which is about 20 mu from B. You also have a mutant stock of flies which carries recessive alleles in all 5 genes and in which chromosome 2 looks like this:

a—o—d—c—b—e

You cross the wild type and your mutant stock to create a heterozygous line. You then do a test cross of a heterozygous fly with one of the mutant stock flies. List all of the phenotypes you expect to see in descending order of frequency, as near as you can estimate.

And here's the answer.

The key here is to recognized that the inversion will prevent any crossover events within the inversion from appearing. The only places where detectable inversions will occur are

A—x—BCD—x—E

This greatly simplifies the problem because you can just treat BCD and bcd as one inseparable block. If you were sitting there trying to enumerate all possible combinations of 5 different pairs of alleles, you were completely missing the point. (Hint: exam questions won't have that kind of tedious, plodding, repetitious work in them. If you find yourself trying to chip away at a problem with brute force, you're missing something important in the question.)

Most common phenotypes: noncrossover
ABCDE
abcde

Next most common: single crossover between A and B
Abcde
aBCDE

Next: single crossover between D and E
ABCDe
abcdE

Least common: double crossover
AbcdE
aBCDe