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A colour blind man marries a woman with normal sight who has no history of colour blindness in her family. What is the probability of their grandson being colour blind?

1. 1
2. Nil
3. 0.25
4. 0.5

Correct answer: 0.25

Solution

Colour blindness is typically X-linked recessive. A colour blind father passes his affected X chromosome to all daughters, making them carriers if the mother is unaffected; a carrier daughter then has a 1/2 chance of passing it to a son, and only sons who receive it are colour blind.

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