If a haemophilic man marries a woman which is carrier (heterozygous) for haemophilia, what would be the possibility that their son would be haemophilic ?

50%

The correct answer is Option (2) - 50%

Haemophilic Man (Father): The father, being haemophilic, has a genotype of X^hY, where X^hY represents the gene for haemophilia. Since males have one X chromosome and one Y chromosome, he can only pass the Y chromosome to his sons.

Carrier Woman (Mother): The mother is a carrier of haemophilia, meaning she carries one normal X chromosome and one X chromosome with the haemophilia gene (X^h). So her genotype is XX^h.

Sons: The sons inherit one X chromosome from their mother (either X or X^h) and the Y chromosome from their father. So, they can either be X^hY (haemophilic) or XY (normal). Since the X^hY genotype leads to haemophilia, there's a 50% chance that their sons will be haemophilic.

Daughters: The daughters inherit one X chromosome from each parent. They can be XX^h (carrier), or X^hX^h (haemophilic). In this case, they have a 50% chance of being carriers XX^h and a 50% chance of being haemophilic X^hX^h . So, the overall probability that their son would be haemophilic is 50%.