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Question
Persons suffering from sickle cell anaemia are at an advantage in Malaria infested areas. Explain .
Solution
Sickle-cell Anaemia: Sickle-cell anaemia is a genetic disorder of human beings, found specially in Blacks. In a sickle-cell anaemic person, normal haemoglobin (HbA) is replaced by HbS whose oxygen-carrying capacity is less than HbA. The RBCs in this disease become, sickleshaped in venous blood owing to the lower concentration of oxygen. This causes rupture of RBCs and severe haemolytic anaemia. Individuals homozygous for abnormal haemoglobin(HbsHbs) die at an early age. In heterozygotes (HbAHbs), the RBCs containing Hbs become sickle-shaped and unable to bind oxygen, and with HbA remain normal.
Why has this character not been eliminated from human population by natural selection? The geographical distribution of sickle-cell anaemia provides answer to the above question. It is found in tropical Africa where malaria is widespread. Malarial parasites that live in RBCs are unable to grow in sickle-shaped RBCs. It means individuals heterozygous for sickle-celled gene are able to cope with malarial infection whereas the normal person with normal RBCs suffers from severe malarial infection in malaria infested areas. This shows that natural selection favoured the sickle-celled character and gene controlling is fixed by natural selection, because of its survival value in malaria-infested regions. Therefore, this character is found in blacks living in malariainfested belt of the world.
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