1. The eye color of wild-type Drosophila flies is red. Different mutations in a
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Question
1. The eye color of wild-type Drosophila flies is red. Different mutations in a single or in multiple genes lead to flies with white eyes. Mutations in the w gene on the X chromosome lead to white eyes instead of the normal red. You have isolated both a white-eyed mutation (designated W-1) that gives a dominant phenotype, and a white-eyed mutation (designated w-2) that gives a recessive phenotype.
(a) A white-eyed male from the W-1 line is crossed to a wild-type female. What color eyes will the female progeny from this cross have?
(b) What color eyes will the male progeny from the cross in part (a) have?
(c) One of the female progeny from the cross in part (a) is mated to a white-eyed male from the w- 2 line. What fraction of the white-eyed progeny from this cross will be female?
2. The body color of Drosophila adult flies is regulated by multiple genes, including the autosomal ebony gene. Ebony body (e) is recessive to the wild type (yellowish) body (e+). A second gene, which is X-linked, controls bristle shape. Normal bristle shape (f+) is dominant to the mutant forked bristles (f). A female with forked bristles that is heterozygous for the body color gene is mated with an ebony body male (his bristles are normal).
(a) What are the genotypes of the parents?
(b) Using a Punnett square, determine the expected proportions of each possible progeny phenotype. Since there is a sex-linked gene here, be sure to include consider sex as part of the phenotype.
3. Describe in your own words what a balancer chromosome is and why it is important in fly genetics (you will have to do some research for this one!)
Explanation / Answer
1. Let's consider that the X chromosome on the mutants of w-1 line will be represented by Xw, with W-1 line as XW and wild-type X chromosome will just be represented as X. Now let's perform the crosses given:
a. White-eyed male from W-1 crossed with wild type female-
The cross will be between XWY x XX
The resulting genotype for progeny from the cross will be XWX and XY.
The female progeny from this cross will all be white-eyed due to the presence of dominant mutation on the X chromosome.
b. The males from the cross done above will all be red-eyed.
c. Female progeny from part a crossed with white-eyed male from w-1 line-
The cross will be between XWX x XwY
The resulting genotype for progeny from the cross will be XWXw, XWY, XY and XwX. 50% or 1/2 of the females from the white-eyed progeny in this cross will be white-eyed due to the presence of dominant mutation on the X chromosome.
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