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?

• A. Sex-linked traits are never seen in girls.
• B. The allele is carried on the Y chromosome.
• C. Nondisjunction occurs in males but not in females.
• D. A sex-linked allele cannot be passed from mother to daughter.
• E. To express an X-linked recessive allele, a female must have two copies of the allele, one of which is contributed by the father.