A woman carrying one haemophilia allele marries an unaffected man. Their sons have a probability of being affected of:

1. Write the mother as X^H X^h and the father as X^H Y. 2. The mother passes either X^H or X^h, each with probability 1/2. 3. A son receives the Y from his father, so his single X comes from his mother. 4. So half of the sons receive X^h and, having no second X to mask it, are affected. 5. The probability among sons is therefore 1/2. Across all children it would be 1/4, which is the value often given by mistake. _Source: NECTA ACSEE 2023 Biology 133/2, Question 4: Genetics_