When a colour blind woman marries a normal man, what is the probability of their son being colour blind?

1. Analyze the question Colour blindness is an X-linked recessive trait. The mother is colour blind and the father is normal. We want the probability that a son is colour blind.

2. Concept

• Genes for colour vision are carried on the X chromosome.
• Females are XX, so they need two recessive alleles (XᶜXᶜ) to be colour blind.
• Males are XY, so a single recessive allele (XᶜY) makes them colour blind.
• A son always receives his X from his mother and his Y from his father.

3. Solution

• Colour blind woman: XᶜXᶜ
• Normal man: XᴺY

Punnett square:

| | Xᶜ | Xᶜ | |---|---|---| | Xᴺ | XᴺXᶜ | XᴺXᶜ | | Y | XᶜY | XᶜY |

Offspring:

• Daughters: XᴺXᶜ, so they are carriers with normal vision.
• Sons: XᶜY, so they are colour blind.

Every son inherits the mother's Xᶜ, and the father gives only a Y, which carries no colour vision gene. So all sons (100%) are colour blind.

4. Answer The probability of a son being colour blind is 100%, which is option B.

Note: if the question had asked about all children, the answer would be 50%, because the daughters are carriers and not colour blind. Here the question asks specifically about sons, so the answer is 100%.

5. Exam tip Remember "mother to son": a son's X chromosome comes only from his mother. If the mother is XᶜXᶜ, all her sons are affected. Always check whether the question asks about sons, daughters, or all children.