Dr. O. Sophila, the boss in your laboratory, has just heard that the laboratory run by Dr. Ara B. Dopsis is proposing that the Drosphiila autosomal genes for eye color and bristle length are linked. Your lab has all the necessary pure-breeding stocks of Drosophila to test the genetic linkage hypothesis of Dr. Dopsis. In Drosophila, red eyes (r+) are dominant to brown eyes (r), and long bristles (t+) are dominant to short bristles (t). Dr. Sophila asks you to design an experiment to test the genetic linkage of eye color and bristle length genes. He tells you to begin by crossing a pure-breeding line homozygous for red eyes and short bristles to a pure-breeding line that has brown eyes and long bristles.A. Give the genotypes of the pure-breeding parental flies, and the genotype(s) and phenotype(s) of the F1 progeny they produce. (5 points)
Parents: r+r+ tt (red, short) x rrt+t+ (brown, long) with F1 that are all r+t/rt+ and have the red, long phenotype.
B. In your experimental design, what is the genotype and phenotype of the line you propose to cross to the F1 to obtain the most useful information about genetic linkage between the eye color and bristle length genes? Explain why you make this choice. (5 points)
The correct cross is a testcross to a brown, short fly with genotype rrtt. The test cross will assure that gamete genotypes produced by the F1 can be identified unambiguously.
C. Assume the eye color and bristle length genes are separated by 20 map units. What are the approximate frequencies of phenotypes expected from the cross you proposed in part B) of the question? Show your work. (5 points)
The expected phenotypes are,
Red, short (r+t/rt) = (0.80)(.5) = 0.40
Brown, long (rt+/rt) = (0.80)(.5) = 0.40
Red, long (r+t+/rt) = (0.20)(.5) = 0.10
Brown, short (rt/rt) = (0.20)(.5) = 0.10
D. How would the results of the cross you propose in part B) differ if the eye color and bristle length genes assort independently? (5 points)
If these genes assort independently, the expected phenotype ratio is 1:1:1:1 (25% each).
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