Why are sex-linked traits commonly passed from carrier mothers to their sons?

• A. Sons inherit the X chromosome from their mother.
• B. Daughters inherit the X chromosome from their father.
• C. X-linked traits are always inherited from fathers.
• D. Sons inherit the Y chromosome from their mother.

Answer: (A)

This question is about how X-linked traits are inherited. Boys have XY chromosomes, so they inherit their single X from their mother and their Y from their father. If the mother is a carrier of an X-linked trait, one of her two X chromosomes carries the allele for that trait. When a son is formed, there’s a good chance he will receive that mutated X from his mother, which is why sex-linked traits often appear in sons while the father’s X does not come to them. The father always passes his Y to sons, not his X, so the son's X always comes from the mother. Daughters can inherit X from both parents, which is why inheritance patterns differ between the sexes.