Question

The genes for sex-linked traits are carried on either the X or Y chromosome. Duchenne muscular dystrophy (DMD) is a recessive, sex-linked disease that is carried on the X chromosome. A woman with DMD and a man without the disease have a child. Which statement best describes the likelihood of the disease passing to their child?
a. There is a 100 percent chance that their daughter will have the disease.
b. There is a 50 percent chance that their son will have the disease.
c. There is a 50 percent chance that their daughter will have the disease.
d. There is a 100 percent chance that their son will have the disease.

Answer 1

Answer:

D

Explanation:

Answer 2

There is a 50% probability that the child will be a boy. If this is the case, he will inherit the X chromosome from his mother and the Y chromosome from his father. Since the woman has DMD, the boy will certainly inherit the disease, since he only has one X chromosome and it carries the disease. We know that both of the mother's X chromosomes carry the disease because since it is recessive, both of her chromosomes must have it in order for her to have it. 

There is also a 50% probability that the child is girl. If this is the case, she will inherit a normal X chromosome from her father and the disease-carrying X chromosome from her mother. Since the disease is recessive, she will not be affected by the disease, since she has one healthy X chromosome and one disease-carrying one. 

If the child is a girl, there is a 0% chance that she will have the disease (although she will be a carrier). 

If the child is a boy, there is a 100% chance that he will inherit the disease. 

The answer is D)