WAEC Biology 2011 Papers

Concept:Sickle cell anaemia is an autosomal recessive disorder, so the disease appears only when both alleles are the sickle cell gene.Explanation:Each person inherits two alleles of the haemoglobin gene, one from each parent.The normal allele is $\mathrm{Hb}^A$ and the sickle cell allele is $\mathrm{Hb}^S$.A person who has two sickle cell alleles has the genotype $\mathrm{Hb}^S\mathrm{Hb}^S$.Since the sickle cell allele is recessive, the presence of two copies means no normal $\mathrm{Hb}^A$ allele is present.Therefore, the defective haemoglobin is fully expressed and the individual shows symptoms of the disease.Such a person is homozygous for the sickle cell gene, not merely a carrier.A carrier would have one normal and one sickle allele, $\mathrm{Hb}^A\mathrm{Hb}^S$, and would not usually show symptoms.Thus, having two sickle cell alleles directly means the person has sickle cell anaemia.Answer:C. has the disease