If a haemophilic man marries a woman who is a carrier (heterozygous) for haemophilia, what is the possibility that their son will inherit haemophilia?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 50%
Haemophilia is a sex-linked recessive disease: its gene lies on the X chromosome. A son always gets his Y chromosome from the father and his only X chromosome from the mother, so whether a son is haemophilic depends on the X he receives from his mother.
Father (haemophilic) = XhY; mother (carrier) = XXh.
| X | Xh | |
|---|---|---|
| Xh | XXh (carrier daughter) | XhXh (haemophilic daughter) |
| Y | XY (normal son) | XhY (haemophilic son) |
Of the two possible sons, one (XY) is normal and one (XhY) is haemophilic, so the chance that a son is haemophilic is 50%.
Option (1) 75% — Incorrect. 75% would need three of every four sons to receive Xh, but the carrier mother passes Xh to only half of her sons.
Option (3) 0% — Incorrect. 0% would be the answer if the mother were normal (XX). Here she is a carrier, so half her sons get her Xh.
Option (4) 100% — Incorrect. 100% would need a mother with XhXh. The father's Xh goes only to daughters, never to sons.
NCERT Extract:
Mendelian Disorders – “… Haemophilia : This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny has been widely studied. In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this, in an affected individual a simple cut will result in non-stop bleeding. The heterozygous female (carrier) for haemophilia may transmit the disease to sons.”