Biology · Principles of Inheritance and Variation · NEET
Haemophilia is recessive and the gene is on the X chromosome, so it is called X-linked (sex-linked) recessive. Recessive means the disease shows only when there is NO normal copy of the gene to cover for it. Because the gene is on the X (not the Y and not an autosome), males and females inherit it differently. NCERT (Class XII, Ch 4) directly calls it a 'sex linked recessive disease'. In NEET, if an option says haemophilia is dominant or Y-linked, it is wrong.
A male is XY, so he has only ONE X chromosome. If that single X carries the faulty haemophilia gene, he has no second X to provide a normal copy, so he is affected. A female is XX, so she has TWO X chromosomes. She becomes affected only if BOTH X's carry the faulty gene, which is rare. With one faulty and one normal X, she is a healthy carrier. This is why haemophilia is far more common in males. This 'one X, no backup' idea is a very common NEET reasoning question.
A carrier female has one normal X and one faulty X (she is heterozygous). She does NOT have haemophilia because her normal X still makes the clotting protein. But she can pass the faulty X to her children. On average, half her sons get the faulty X and become affected, and half her daughters become carriers. NCERT specifically describes transmission 'from unaffected carrier female to some of the male progeny'. Remember: carrier = looks normal but can pass the gene.
The clotting-factor gene is physically located on the X chromosome, not the Y chromosome. The Y chromosome is small and carries mostly male-determining genes, not this clotting gene. So the trait follows the X, which both sexes can carry. A useful check: a Y-linked trait would pass only from father to ALL sons, but haemophilia clearly passes through carrier mothers to sons, so it must be X-linked. In NEET 2018, an option matching 'Haemophilia - Y linked' was marked WRONG.
Mother is X^H X^h (carrier), father is X^H Y (normal). Their children: daughters get one X from mother and X^H from father, so they are either X^H X^H (normal) or X^H X^h (carrier) - none are affected. Sons get their single X from the mother and Y from the father, so they are either X^H Y (normal) or X^h Y (affected). Result: about half the sons are affected, no daughters are affected. This exact pattern is why NEET keeps asking about carrier mothers.
In haemophilia, a single protein in the blood-clotting cascade is defective (this is clotting factor VIII or IX). Blood clotting works like a chain of proteins switching each other on. If one protein in that chain is missing or faulty, the chain breaks and a clot cannot form properly. So even a small cut leads to long, non-stop bleeding. NCERT says 'a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected.'
Both are X-linked recessive, so the inheritance PATTERN is the same: more males affected, passed through carrier mothers. The difference is only in WHAT goes wrong. Colour blindness affects the cone cells / colour-vision genes in the eye. Haemophilia affects a blood-clotting protein. So for NEET, if a question asks about the inheritance type, both give the same answer (X-linked recessive); if it asks about the defect, they differ.
Which of the following most appropriately describes hemophilia?
Select the correct match.
A woman has an X-linked condition on one of her X chromosomes. This chromosome can be inherited by
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes, but it is rare. A female must inherit the faulty X from BOTH parents (X^h X^h), meaning her father is haemophiliac and her mother is at least a carrier. With only one faulty X she is a healthy carrier, not a patient.
No. A father gives his Y chromosome (not his X) to a son, and the haemophilia gene is on the X. So an affected father cannot pass haemophilia to his sons - but he passes his faulty X to ALL his daughters, making them carriers.
It is a Mendelian (single-gene) disorder, because it is caused by one faulty gene on the X chromosome. It is NOT a chromosomal disorder like Down's syndrome, which is caused by a change in chromosome number.
Classic haemophilia involves a defect in a single clotting-cascade protein - factor VIII (haemophilia A) or factor IX (haemophilia B). For NEET, NCERT only says 'a single protein of the clotting cascade' is affected, so knowing it is one clotting protein is enough.
It is well studied through the royal family of Queen Victoria, where carrier females passed it to several male descendants. NEET may mention it as the classic example of an X-linked recessive disease transmitted through carrier females.
