Sickle cell anaemia is caused by defective hemoglobin gene. the persons homozygous for sickle cell anaemia condition have two abnormal genes, one on each homologous chromosome .such persons are likely to die in their childhood because of large number of abnormal RBCs. on the other hand persons heterozygous for sickle cell anaemia have 1 normal gene at 1 homologous chromosome and one abnormal gene at other homologous chromosome .such persons have both normal and abnormal RBCs. Abnormal RBCs become sickle shaped under low oxygen conditions . Heterozygotes have more chances of survival up to reproductive age. It has been found that the persons heterozygous for sickle cell trait can tolerate the infection of Malaria caused by plasmodium falciparum .actually the sickle shaped RBCs do not support erythrocytic schizogony of malaria parasite plasmodium falciparum. Hetrozygotes therefore do not suffer seriously from plasmodium falciparum infection. they have normal RBCs also which can carry oxygen. In certain parts of Africa where infection due to plasmodium falciparum is common sickle cell trait is also prevalent. the natural selection selected sickle cell trait because the heterozygous for this trait were more resistant than normal persons against malarial infection. this existence of two different types of traits or say alleles of a gene in a population is called balanced polymorphism and this is due to the selection of heterozygotes by nature because of selective advantage in survival.
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