Read the following passage and answer the questions that follow : Haemophilia is a sex-linked recessive disease. Study the pedigree analysis given below showing the inheritance of the disease in a family and answer the questions that follow :
(b) (i) The possibility of a human female becoming a haemophilic is extremely rare. Explain.
(ii) Why do the sons of a haemophilic father never suffer from this disease ?
Show Hint
Females need two recessive alleles, which is rare; sons receive the Y chromosome, not the X, from their father.
Step 1: Part (i): why a haemophilic female is rare:
A female has two X chromosomes. She is affected only if both carry the recessive allele (\( \mathrm{X^{h}X^{h}} \)).
This needs a carrier or affected mother and an affected father.
Such a mating is unlikely because haemophilic males are few. A homozygous recessive female may even die early in development.
Step 2: Part (ii): sons of an affected father:
A son gets his Y chromosome from the father and his X chromosome from the mother.
The father gives his X only to his daughters.
So a son does not receive the father X carrying the haemophilia allele, and he is not affected by it unless the mother is a carrier.
Final Answer:
(i) A female must be \( \mathrm{X^{h}X^{h}} \), which needs a carrier mother and a haemophilic father, a very rare mating. (ii) Sons get the Y chromosome from the father, not his X.
