Question

In Drosophila, the red-eye allele for eye color exhibits complete dominance over the recessive white-eye allele. In some of Thomas Hunt Morgan's initial experiments, he crossed red-eyed female flies with white-eyed males. When Morgan crossed the F1 red-eyed heterozygous females with red-eyed males, he found that half of the male progeny were white-eyed. What is the most likely explanation of this result?
1. The gene responsible for eye color is on the Y chromosome.
2. The gene responsible for eye color is on the X chromosome.
3. The gene responsible for eye color is on an autosome.
4. Chemical factors secreted by males changed the color of the eye.
5. Chemical factors secreted by males mutated the eye color gene.

Answer 1

Answer:

2. The gene responsible for eye color is on the X chromosome.

Explanation:

For X-linked genes, a male can either be affected or unaffected because the Y chromosome is hypothesized not to carry any gene. Females on the other hand can be a carrier, affected or unaffected because the two sex chromosomes carry genes.

If eye color allele in drosophilia is represented by R, the cross between F1 red-eyed heterozygous female and red-eyed males can be illustrated below:

Red-eyed heterozygous female = $X^RX^r$

Red-eyed male = $X^RY$

$X^RX^r$   x   $X^RY$

Progeny: $X^RX^R, X^RY, X^RX^r, X^rY$

Out of the 2 male progeny from the cross,  1 ($X^rY$) is white-eyed while the remaining one ($X^RY$) is red-eyed

Hence, the result shows that the gene for eye color in Drosophilia is X-linked.

Option 2 is the correct answer.