Human Heredity 9th Edition by Michael Cummings

Exercise 9

Assume that humanlike creatures exist on Mars. As in the human population on Earth, there are two sexes and even sex-linked genes. The gene for eye color is an example of one such gene. It has two alleles. The purple allele is dominant to the yellow allele. A purple-eyed female alien mates with a purple-eyed male. All the male offspring are purple-eyed, whereas half the female offspring are purple-eyed and half are yellow-eyed. Which is the heterogametic sex? Why?