A woman with normal vision whose father was colour-blind marries a man with normal vision. What is the probability that their son will be colour-blind? (Colour blindness is X-linked recessive)

The woman's father was colour-blind (X^b Y), so she must be a carrier (X^B X^b). Her husband has normal vision (X^B Y). The cross X^B X^b × X^B Y produces sons with genotypes X^B Y (normal) and X^b Y (colour-blind) in equal proportions, giving a 50% probability that any son will be colour-blind. Daughters cannot be colour-blind in this cross.