A true-breeding mutant strain of Drosophila has a mutant eye phenotype in which

ID: 222273 • Letter: A

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A true-breeding mutant strain of Drosophila has a mutant eye phenotype in which the eyes are shaped like kidney beans. When males of this strain are mated to true-breeding wild type females, all of the F1 offspring have wild type eyes. Some of the F1 males and females are mated and 200 F2 offspring are examined for phenotypes with the following results: 152 with wild type eye shape (78 males, 74 females) and 48 with kidney bean eye shape (23 males, 25 females). a. Is the eye shape mutation dominant or recessive with respect to the wild type allele for eye shape? Briefly explain. b. Does the eye shape gene appear to be located on an autosome or a sex chromosome? Briefly explain. c. Using the format for naming mutant and wild type alleles of Drosophila genes (described on p. 109 of the lab manual), assign allele designations and write out the genotypes for the cross of the true-breeding mutant (kidney) eye males and true-breeding wild type females and their F1 offspring. d. When the F1 males and females in 1c are crossed to each other, what are the expected genotypes of their F2 offspring?

Explanation / Answer

Answer:

a. Since F1 offsprings have all wild type eyes, it means that the eye shape mutation is recessive to the wild type one.

b. Because wild type (E) is what shows up, we know that E is dominant.

If this gene was sex-linked (on the X chromosome), then the females in F2 would possess atleast one XE from the males (father), so they would all show up as wild type phenotype. But here we get the both wild type and mutant females in F2. So, we know that the gene must be autosomal.

c. (Since the lab manual has not been provided, the crosses have been given in general)

EE (Females, wild type) * ee (Males, mutant)

F1: Ee (Wild type)

d. Parents: Ee (Wild type females), Ee (Wild type males)

F2: EE, Ee, Ee, ee

So, the expected genotypic ratio = 1 : 2 : 1

Phenotypic ratio = 3 : 1 (Wild type : Mutant)

In the above example, we get a ratio of = 152 (Wild type) : 48 (Mutant) = 3 : 1

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