Question

1) In Drosophila, body colour is controlled by a gene on the X chromosome called yellow. It acts in a recessive manner. It causes the body to appear yellow rather than the normal brown of wildtypes. In a mating between a brown male fruit fly and a brown heterozygous female, what percentage of the male offspring is expected to be brown? Explain how you determined the percentage, specifying the genotypes of the parents and all potential offspring. For this question, use B to designate the dominant wildtype allele and b to designate the recessive mutant allele.

2) in a mating between a brown male fruit fly and a yellow female fruit fly, what percentage of the female offspring will be yellow?  Explain how you determined the percentage, specifying the genotypes of the parents and all potential offspring. Use B to designate the dominant wildtype allele and b to designate the recessive mutant allele.

For this and the following questions, use the forms X^AX^a , X^AY, etc, replacing "A" with the letter specified in each question.

Answer 1

1)

The recessive trait is expressed when only recessive alleles are present for that gene. Here,  yellow body color is a recessive trait.

The genotype of male brown fly is: XBY

The genotype of brown heterozygous female: XBXb

The punnet square for the cross is given below:

Here,

XBXb - Heterozygous female brown drosophila

XBXB - Homozygous female brown drosophila

XBY - Brown male drosophila

XbY - Yellow male drosophila

The ratio of the male offspring to be brown is 1:1. Hence, the percentage of the male offspring is expected to be brown is 50%.

2)

The genotype of male brown drosophila fly : XBY

The genotype of yellow female drosophila fly : XbXb

The punnet square for the cross is given below:

XBXb - Heterozygous female brown drosophila

XbY - Yellow male drosophila

Here, all the female offspring will have a body color of brown, since one dominant allele is inherited from the male parent with brown body color and yellow body color is a recessive trait. Hence, the percentage of the female offspring will be yellow is zero.