WAEC Biology 2010 Papers

Concept:Haemophilia is an X-linked recessive disorder, so a son's X chromosome always comes from his mother.Explanation:The normal male has genotype $X^H Y$, and the carrier woman has genotype $X^H X^h$.A male child inherits the $Y$ chromosome from his father and one $X$ chromosome from his mother.The carrier mother produces two types of eggs: one carrying $X^H$ and the other carrying $X^h$.There is an equal probability, $\frac{1}{2}$ or $50\%$, that a son receives either allele from his mother.If the son receives the $X^h$ allele, he will be haemophilic because the $Y$ chromosome has no corresponding normal allele to mask the defect.Therefore, each male child of this couple has a $50\%$ chance of being haemophilic.Answer:B. $50\%$