Which of the following statements about haemophilia is incorrect?

The correct answer is Option (1) → This is a sex-linked dominant disease.

Haemophilia is a sex-linked disorder: the faulty gene is on the X chromosome and is recessive. A female with one faulty X is usually a healthy carrier. A male has only one X, so one faulty allele gives him the disease.

Option (1) This is a sex-linked dominant disease. — Incorrect statement about haemophilia. Haemophilia is sex-linked recessive, not dominant. That is why carrier females show no symptoms.

Option (2) The heterozygous female (carrier) for haemophilia may transmit the disease to sons. — Correct statement about haemophilia. A carrier mother passes the faulty X to some of her sons, and they have no second X to hide it.

Option (3) A single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. — Correct statement about haemophilia. Without this one protein, blood does not clot, so a simple cut causes non-stop bleeding.

Option (4) The possibility of a female becoming a haemophilic is extremely rare. — Correct statement about haemophilia. Her mother must be at least a carrier and her father haemophilic, and such fathers rarely survive into later life.

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. The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at least carrier and the father should be haemophilic (unviable in the later stage of life).”