Question

A man and a woman walk into a genetic counselor’s office. The man’s brother has recently been diagnosed with Duchenne muscular dystrophy. The woman has no history of the illness in his family.

a) They are concerned about having an affected child themselves. The woman is pregnant with a girl. Diagram this cross. What is the chance that the child the woman carries will have the disorder?

a) If they have additional children, would a sperm or egg donor help to decrease the chance of an affected child?

Answer 1

Duchenne muscular dystrophy is an x-linked recessive disorder.
WT dominant allele = XD
Mutant recessive allele = Xd

Since males are hemizygous, one defective allele can lead to the development of the disease.
Females with one defective allele are carriers.

It is given that the man's brother is affected. So, both of his parents must carry at least one recessive allele each.

Parents of the man: XDXd X XDY
Progeny: XDXD XDY XDXd XdY

Man X woman: XDY X XDXD
Progeny: XDXD XDXD XDY XDY

If the man does not show any disease symptoms, he must be XDY. Since he does not carry any disease-causing allele, he can not transmit it to his progeny.
So, the probability of their child to be affected = 0
Since the probability is zero, there is no need for donors.