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.
WBBSE
Class X
Life Science
03 (b) Some Common Genetic Diseases
02 Mark
2025
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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