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.

Answer: (E)

Verified step by step guidance

1. Understand that Duchenne muscular dystrophy is a sex-linked recessive disorder, meaning the gene causing the disorder is located on the X chromosome.
2. Recognize that males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).
3. For a female to express a recessive X-linked disorder, she must inherit two copies of the recessive allele, one from each parent.
4. In the case of Duchenne muscular dystrophy, a female would need to inherit the recessive allele from her mother (who could be a carrier) and her father (who would have the disorder).
5. Since males with Duchenne muscular dystrophy often do not survive to reproductive age, it is rare for a female to inherit the disorder from her father, making the condition almost never seen in girls.