A dominant mutation B results in brown eyes rather than the wildtype red eyes in

Question

A dominant mutation B results in brown eyes rather than the wildtype red eyes in Drosophila. Genotypes BB and Bb therefore have brown eyes, whereas genotype bb has red eyes. The mutation shows complete penetrance and is located on one of the autosomes. A geneticist carries out a cross of homozygous brown (BB) by homozygous red (bb). The following questions pertain to crosses made using the F1 progeny of this cross, all of which have the heterozygous genotype Bb.

a.The F1 male and female progeny are mated together to produce an F2 generation. From the F2 generation, a large number of brown-eyed males, taken at random, are mixed together in a bottle containing homozygous bb females. What genotypes and phenotypes would be expected among the progeny emerging from this bottle, and in what proportions?

b. The F1 male and female progeny are mated together to produce an F2 generation. The brown-eyed F2 males and brown-eyed F2 females are mated at random. What genotypes and phenotypes would be expected among the progeny, and in what proportions?

Explanation / Answer

A. The Cross between F1 gives F2 with the following genotypes. Bb * Bb = BB, Bb, bB, bb. The gametes produced by BB males will be all B, those produced by Bb males will be 1/2 B and 1/2 b.

So, among the brown-eyed males, the proportion of B-bearing gametes = (1/3)(1) + (2/3)(1/2) = 2/3

Similarly, the proportion of b-bearing gametes = (1/3)(0) + (2/3)(1/2) = 1/3.

The cross with bb (homozygous bb females) gives the expected progeny as 2/3 Bb (brown) and 1/3 bb (red).

B. The genotypes of gamets produced by male and female are same i.e. 2/3 B and 1/3 b. The Punnett square cross gives the progeny genotypes as 4/9 BB, 4/9 Bb, and 1/9 bb. Thus the progeny phenotypes are 8/9 brown and 1/9 red eyed.

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