(d) : Sickle cell anaemia is an autosomal hereditary disorder in which the erythrocytes become sickle shaped. The disorder or disease is caused by the formation of an abnormal haemoglobin called haemoglobin­-$S$. As found out by Ingram $(1958)$, haemoglobin­-$S$ differs from normal haemoglobin-$­A$ in only one amino acid ­$ 6^{th}$ amino acid of $\beta$-­chain, glutamic acid, is replaced by valine. This is the major effect of the allele. During conditions of oxygen deficiency $6$-­valine forms hydrophobic bonds with complementary sites of other globin molecules. It distorts their configuration. As a result, erythrocyteshaving haemoglobin-$S$ become sickleshaped.Carriersof the sickle cell anaemia gene are protected againstmalaria because of their particular haemoglobinmutation this explains why sickle cell anaemia isparticularly common among people of African origin.The malarial parasite has a complex life cycle andspends part of it in red blood cells and feeds onhaemoglobin. Both sickle-cellanaemia and thalassemiaare more common in malaria areas, because thesemutations convey some protection against the parasite.In a carrier, the presence of the malaria parasite causesthe red blood cell to rupture, making the Plasmodiumunable to reproduce. Further, the polymerisation of $Hb$ affects the ability of the parasite to digest $Hb$ in the firstplace. Therefore, in areas where malaria is a problem,people’s chances of survival actually increase if theycarry sickle cell anaemia. Thus, sickle-cell anaemia is apotential saviour from malaria.