A woman who is a carrier for colour blindness marries a man with normal colour vision. The probability that any son of theirs is colour blind is
- A. 0
- B. one in four
- C. one in two
- D. certain
Explanation
A son takes his single X from his mother, and half of her X chromosomes carry the defective allele, so half the sons are colour blind. The answer of one in four applies to the different question of the chance that a child of either sex is colour blind, which is the usual mistake here. None of the daughters is colour blind, since each receives a normal X from the father, but half are carriers.