40. 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. To express an X-linked recessive allele, a female must have two copies of the allele.
e. A sex-linked allele cannot be passed from mother to daughter.

Question

40.	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.	To express an X-linked recessive allele, a female must have two copies of the allele.
e.	A sex-linked allele cannot be passed from mother to daughter.

alphadxg · Answer

D - because males will get one Y from the Dad, and so it has a 50% chance of getting a either a dominant or recessive .