Duchenne muscular dystrophy is caused by a sex-linked recessive allele. Its victims are almost invariably boys, who usually die before the age of 20. Why is this disorder almost never seen in girls?

If a woman is heterozygous for a recessive sex-linked allele and her husband has the dominant allele, which one of the following is true about the probabilities for their children?

In werewolves (hypothetically), the pointy ear allele (P) is dominant over the round ear allele (p). The gene for ear shape is on the X chromosome. (Sex determination in werewolves is the same as for "other" humans.) A certain female werewolf has pointy ears even though her father had round ears. What percentage of her sons will have round ears if she mates with a werewolf with round ears?

A woman is red-green color-blind. What can we conclude, if anything, about her father?

A woman is a carrier for red-green color blindness, a sex-linked trait. Her husband is normal (not color-blind) for this trait. What are the chances that their newborn daughter will be red-green color-blind?

Hemophilia is an X-linked disorder. The daughter of a father with hemophilia and a carrier mother has a __________ probability of having hemophilia.

Consider a woman who is a carrier of a recessive X-linked allele for hemophilia. Which of the following possibilities could explain the presence of the hemophilia allele in her genotype?

In a certain fish, fin rays (supporting structures for the fins) can be either bony or soft in adult fish. Sex linkage in a fish is similar to that in humans. What evidence would most strongly support the idea that the ray locus is on the X chromosome?

X-linked genes differ from Y-linked genes in which of the following ways?