Question

Why does the son of a carrier mother and a normal father suffer from haemophilia whereas the son of a haemophiliac father and normal mother would not?

Answer 1

The reason why the son of a carrier mother and a normal father suffers from haemophilia, whereas the son of a haemophiliac father and a normal mother does not, lies in the nature of sex-linked inheritance, specifically X-linked recessive inheritance of haemophilia:

1. Haemophilia is a sex-linked recessive disorder caused by a gene located on the X chromosome.
2. A carrier mother has one normal X chromosome and one X chromosome carrying the haemophilia gene (X^h). She is phenotypically normal but can pass the defective X chromosome to her children.
3. A normal father has one normal X chromosome and one normal Y chromosome (XY).
4. Sons inherit their X chromosome from their mother and Y chromosome from their father.
   • If the mother is a carrier (X X^h) and the father is normal (XY), the son has a 50% chance of inheriting the defective X^h from the mother, resulting in haemophilia because males have only one X chromosome (no second X to mask the defective gene).
5. If the father is haemophiliac (X^hY) and the mother is normal (XX), all sons will inherit the Y chromosome from the father and one normal X chromosome from the mother, thus they will be normal. The diseased X chromosome from the father cannot be passed to sons (because sons inherit Y from the father) but will be passed to daughters, making them carriers.

Hence, the transmission of haemophilia is from carrier mothers to sons, not from haemophiliac fathers to sons.