Question

Why is a large population in African countries carrier of sickle cell anaemia? What happens when both parents are carriers of the disease? What is the cause of this disease? In which condition is it fatal? Explain with the help of chart showing phenotypes and genotypes.

Answer 1

The sickle-cell allele is relatively common in some malaria-endemic regions because heterozygous carriers have increased resistance to severe malaria. The disease is caused by a single base substitution in the β-globin gene, changing glutamic acid to valine in the haemoglobin chain. Severe disease is associated with the homozygous Hb^sHb^A condition.

If both parents are carriers: Hb^AHb^s × Hb^AHb^s

Genotype	Phenotype	Probability
HbAHbA	Unaffected, non-carrier	25%
HbAHbs	Carrier; usually mild effects and increased malaria resistance	50%
HbsHbs	Sickle-cell anaemia; severe disease	25%