WBBSEClass XLife Science03 (b) Some Common Genetic Diseases02 Mark2025 Moderate

QQuestion

If an apparently normal female marries a normal male and if two sons are born to them then show with the help of a cross the probability of haemophilia in the sons.

Exam-ready answer 02 Mark

Answer

Haemophilia is an X‑linked recessive disease; an “apparently normal” mother may be a carrier (XᴴXʰ), father is normal (XᴴY).

Parental genotypes:

Female (carrier): XᴴXʰ

Male (normal): XᴴY

Gametes:

Female: Xᴴ, Xʰ

Male: Xᴴ, Y

Punnett cross:

Xᴴ (father) Y (father)
Xᴴ (mother) XᴴXᴴ
(Normal daughter)
XᴴY
(Normal son)
Xʰ (mother) XᴴXʰ
(Carrier daughter)
XʰY
(Haemophilic son)

Among sons only (XᴴY,  XʰY), probability of haemophilia in sons = 1 out of 2 = 50%.

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