Dear sickle cell trait confers selective advantage in a malaria endemic areas and has resulted in no control of the trait not minding all genetic counseling.
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Br J Haematol. 2017 Nov;179(4):543-556. doi: 10.1111/bjh.14886. Epub 2017 Aug 23. The role of the red blood cell in host defence against falciparum malaria: an expanding repertoire of evolutionary alterations.
Sickle cell anaemia is an adaptation to cope up with malaria in malaria endemic areas of the world. It is an hereditary disease that causes the production of abnormal haemoglobin as a result, the shape of the red blood cells (RBCs) containing it becomes distorted and sickle shape. In malaria endemic regions the sickling trait in heterozygous form experiences a much milder and far less lethal form of malaria than those who do not carry the trait. Heterozygous individuals produce both normal and abnormal haemoglobin are naturally protected as the malarial parasite Plasmodium cannot live in distorted or sickled cells.