In fruit flies, the gene for eye color is located on the X chromosome, and the red eye allele (R) is dominant
to the white eye allele (r). A carrier female is mated with a male fly with genotype X
rY. Which of the
following statements best describes the expected outcome of the cross?
a. The chance of an offspring having red eyes is 75%
b. The chance of an offspring having white eyes is 50%
c. The chance that a male offspring will have white eyes is 0%
d. The chance that a female offspring will have red eyes is 100%

So the genotype of the male fly is X^r Y. This means the male has white eyes because he only has one X chromosome, so he will express whatever allele is on that chromosome.

The genotype of the female fly, though, is X^R X^r. This is because carriers are heterozygous for traits, and the sex chromosomes of most females is XX (with some exceptions, like XXX, or X (Turner syndrome)).

If we cross this, what are the outcomes? Do a Punnett square. You get the genotypes of X^r X^R, X^r X^r, X^R Y, and X^r Y. The expected phenotypes are a red eye female, white eye female, red eye male, and white eye male.

Now, let's look at the choices:

A.: The chance of an offspring having red eyes is not 75%; it is 50%.

B.: The chance of an offspring having white eyes is 50%!!!

C.: The chance that a male offspring will have white eyes is 50%, not 0%.

D.: The chance that a male offspring will have white eyes is 50%, not 100%.

Sooo, it's B!