If a boy exhibits a sex-linked trait that is carried on the X chromosome, what must be true?
A. His mother exhibited that trait.
B. His father exhibited that trait.
C. His mother carried at least one allele for that trait.
D. His father’s mother carried an allele for that trait.

Question

If a boy exhibits a sex-linked trait that is carried on the X chromosome, what must be true?
A.  His mother exhibited that trait.
B.  His father exhibited that trait.
C.  His mother carried at least one allele for that trait.
D.  His father’s mother carried an allele for that trait.

anonymous · Answer

If this is all the information given then I think it must be C. If the mutated gene is recessive then the mother could be a carrier and not show any signs of the disease because of the othe allele on the other X chromosome. The father need not show the disease for the son to inherit a recessive allele. Haemophilia is a good example if you want to understand the inheritance more deeply.