WAEC Biology 2013 Papers

Concept:Haemophilia is an X-linked recessive genetic disorder.Explanation:The affected father has the genotype $\mathrm{X}^h\mathrm{Y}$.The carrier mother has the genotype $\mathrm{X}^H\mathrm{X}^h$.Their possible children are:Sons: $\mathrm{X}^H\mathrm{Y}$ (normal) and $\mathrm{X}^h\mathrm{Y}$ (haemophilic).Daughters: $\mathrm{X}^H\mathrm{X}^h$ (carrier) and $\mathrm{X}^h\mathrm{X}^h$ (haemophilic).So, out of 4 possible children, 2 are haemophilic.Percentage affected: $\frac{2}{4} \times 100 = 50\%$.Answer:50% of their children are likely to be haemophilic.Correct option: B. 50%.