Sickle cell anaemia has not been eliminated f | vedclass

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(D) : Sickle cell anaemia is an autosomal hereditary disorder in which the erythrocytes become sickle-shaped. The disorder is caused by the formation

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it provides immunity against malaria.

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(D) : Sickle cell anaemia is an autosomal hereditary disorder in which the erythrocytes become sickle-shaped. The disorder 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: the $6^{th}$ amino acid of the $\beta$-chain,glutamic acid,is replaced by valine. Carriers of the sickle cell anaemia gene are protected against malaria because of this particular haemoglobin mutation. This explains why sickle cell anaemia is particularly common among people of African origin. The malarial parasite has a complex life cycle and spends part of it in red blood cells,feeding on haemoglobin. Both sickle-cell anaemia and thalassemia are more common in malaria-prone areas because these mutations convey some protection against the parasite. In a carrier,the presence of the malaria parasite causes the red blood cell to rupture,making the $Plasmodium$ unable to reproduce. Further,the polymerisation of $Hb$ affects the ability of the parasite to digest $Hb$. Therefore,in areas where malaria is a problem,people's chances of survival actually increase if they carry the sickle cell anaemia trait. Thus,sickle-cell anaemia acts as a potential saviour from malaria.

Column-$I$ (Traits)	Column-$II$ (Type of inheritance)
$i$. Roan cattle	$a$. Multiple allelism
$ii$. Pink flowered $Mirabilis \, jalapa$ plant	$b$. Co-dominance
$iii$. Sickle cell anaemia	$c$. Incomplete dominance
$iv$. Wings in $Drosophila$	$d$. Pleiotropism

Sickle cell anaemia has not been eliminated from the African population because

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