Question

Which of the statements cannot occur with X‑linked dominant inheritance for a rare trait?

a) An affected woman can have a normal daughter.
b) An affected woman can pass on the trait to both sons and daughters.
c) An affected man can pass on the trait to his daughter.
d) An affected man can pass on the trait to his son.
e) An affected man can have a normal son.

Answer 1

D) an affected man can pass on the trait to his son

Assuming that this is a dominant X-linked disease:

It is possible for an affected female to have a normal daughter if she is a heterozygote/has one normal allele. She is also able to pass the trait on to her sons and daughters (each would get one X chromosome from her, which could be affected). And affected man would pass the trait to his daughter, as he only has one X chromosome to give (disease carrying). And affected man will have a normal son if the mother is unaffected, as the father only passes on his Y chromosome to his son.

Therefore, the only false answer is that an affected male can pass the trait onto his son. The father will give him a Y chromosome instead of the disease-carrying X. If the son gets the disease, it would be from the mother instead, who provides the X.