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Sex-Linked Inheritance in Human Beings - Haemophilia

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Estimated time: 9 minutes

CISCE: Class 12

Definition: Haemophilia

Haemophilia is an X-linked recessive hereditary disorder in which the blood lacks the normal clotting factor, resulting in prolonged bleeding even from minor injuries.

CISCE: Class 12

Haemophilia

Haemophilia is a recessive disorder.
The gene for haemophilia is located on the non‑homologous region of the X chromosome.
There is no corresponding allele on the Y chromosome, so the recessive gene expresses itself easily in males.
Females are usually carriers and suffer from haemophilia only when both X chromosomes carry the recessive gene.

Genotypes:

Individual	Genotype	Condition
Normal male	XᴴY	Normal blood clotting.
Haemophilic male	XʰY	Haemophilia.
Normal female	XᴴXᴴ	Normal blood clotting.
Carrier female	XᴴXʰ	Carrier for haemophilia.
Haemophilic female	XʰXʰ	Haemophilia (rare).

CISCE: Class 12

Important Crosses

1. Haemophilic male × Normal female

Result:

All daughters: XᴴXʰ → carrier females.
All sons: XᴴY → normal males.
No child is haemophilic in this cross, but all daughters carry the haemophilia gene.

2. Carrier female × Normal male

Result:

XᴴXᴴ → normal daughter.
XᴴXʰ → carrier daughter.
XᴴY → normal son.
XʰY → haemophilic son.

Maharashtra State Board: Class 12
CISCE: Class 12

Key Points: Haemophilia

Haemophilia is an X‑linked recessive hereditary disorder in which blood fails to clot normally, causing prolonged bleeding even after minor injuries.
It occurs due to deficiency of clotting factors VIII or IX, controlled by a recessive gene (Xʰ) on the X chromosome.
Males (XʰY) are more commonly affected; females are affected only when both X chromosomes carry the recessive gene (XʰXʰ); otherwise, they are carriers (XᴴXʰ).
Haemophilia shows criss‑cross inheritance, where the gene passes from an affected father to a carrier daughter and then to a haemophilic grandson.
A haemophilic male × normal female produces carrier daughters, while a carrier female × normal male can produce 50% haemophilic sons.