Question

The possibility of a female becoming haemophilic is extremely rare because the mother of such a female has to be at least (i) and father should be (ii).

• A. (i) Haemophilic, (ii) Carrier
• B. (i) Carrier, (ii) Haemophilic
• C. (i) Haemophilic, (ii) Normal
• D. (i) Haemophilic, (ii) Haemophilic

Answer 1

Answer: (B)

(i) Carrier, (ii) Haemophilic

• Haemophilia is genetically due to the presence of a $recessive$ sex-linked gene h, carried by the X chromosome.
• A female becomes haemophilic only when both X chromosomes carry the gene $\left(XhXh\right)$. A female having only one allele for haemophilia $\left(X{X}^h\right)$ appears normal because of the allele for normal blood clotting present on the other X-chromosome is dominant. Such females are known as carriers.

• In the case of males, a single gene for the defect is able to express itself as the Y-chromosome is devoid of any corresponding allele $\left(X{h}^Y\right)$.
• Thus, the possibility of a human female becoming haemophilic is extremely rare because she has to be homozygous recessive for the trait, i.e., her father must be a haemophilic and mother must be at least a carrier.
• So, the correct answer is '(i) Carrier, (ii) Haemophilic'