Question

In a normal couple, half the sons are haemophilic while half the daughters are carriers. The gene responsible for it is located on

• A. X-chromosome of father
• B. Y-chromosome of father
• C. One X-chromosome of mother
• D. Both the X-chromosomes of mother

Answer 1

The correct option is C One X-chromosome of mother

Haemophilia is a sex linked disease in which the patient continues to bleed even from a minor cut since he or she does not possess the natural phenomenon of blood clotting.

Haemophilia is genetically due to the presence of recessive gene h, carried by X-chromosome. A female becomes haemophiliac only when both its X-chromosomes carry the gene $\left(X^h{X}^h\right)$. However, such females generally die before birth because the combination of these two recessive alleles is lethal.

A female having only one allele for haemophilia $\left(X{X}^h\right)$ appears normal because the allele for normal blood clotting present on the other X-chromosome is dominant. Such females are known as carriers.

In case of males, a single gene for the defect is able to express itself as the Y-chromosome is devoid of any corresponding allele $\left(X^{h}Y\right)$.