JAMB Biology 2018 Paper

Concept:Haemophilia is an X-linked recessive disease, so the inheritance pattern depends on the sex chromosomes.Explanation:Let the normal allele be $X^H$ and the haemophilic allele be $X^h$.The woman is heterozygous: $X^H X^h$.The man is haemophilic: $X^h Y$.Possible gametes from mother: $X^H$ or $X^h$.Possible gametes from father: $X^h$ or $Y$.Offspring combinations:• $X^H X^h$: carrier female, normal phenotype• $X^h X^h$: haemophilic female• $X^H Y$: normal male• $X^h Y$: haemophilic maleSo 2 out of 4 offspring have haemophilia, which is 50%.Answer:C. 50%