Sickle cell anemia disease or trait is found predominantly in people whose ancestral ties are in the sub Saharan Africa; to the rate of 1 out of every 12 African Americans are afflicted. There was a benefit to having the disease.
About Sickle Cell
Sickle cell conditions are inherited diseases that require an afflicted individual to inherit one or two copies of the abnormal hemoglobin gene. In the case of the sickle cell trait, the individual inherits one abnormal gene from a carrier parent, unlike an individual afflicted with full blown sickle cell anemia who inherits two abnormal genes; one from each parent. The abnormal hemoglobin gene causes the production of abnormal hemoglobin, which in turn causes red blood cells to sickle, instead of being their normal discoid shape. The sickle shaped cells aren’t suited to travel smoothly though the blood vessels, and instead the abnormally shaped cells tend to stick together and clog blood vessels, causing the symptoms associated with the condition. People with the sickle cell trait do not normally display sickling cells or symptoms associated with the disease, except under certain conditions of stress which can be caused by an infection or by the blood being under low oxygen concentration.
Malaria is one of the world’s deadliest diseases with about 2 million deaths annually, found prevalently in the hot damp climatic conditions of sub-Saharan African where mosquitoes thrive. Malaria is caused by a plasmodium parasite that is found in the mosquito which inadvertently transmits the parasite to humans as it feeds on human blood. The parasite spends some of its lifecycle in the mosquito and some of it in the body of the human host. Once in the body, the parasite first invades the liver cells then moves into the red blood cells to finish off its lifecycle. The parasites eventually destroy the red blood cells, causing chills and fever.
Malaria and Sickle Cell Link
Epidemiological studies have shown an undeniable link between malaria survival and sickle cell anemia. One would expect natural selection to remove the debilitating gene from a population, but instead the gene survived and became prevalent. This is a case of evolutionary trade-off, where a normally unwanted condition offers immunity against a deadly disease. When a normal individual suffers from malaria, the parasite destroys their red cells, which may eventually lead to death; however, the mutational change in the hemoglobin in a carrier with the sickle cell trait impairs the development and growth of the parasite in its human host. During the stress related to a malarial infection; the person's cells sickle. The liver recognizes the abnormal cells and eliminates them from the blood effectively eliminating the malaria parasite, hence the better malaria survival rate. When Africans came to the US during slavery, having the sickle cell trait was no longer advantageous, and the incidence of the gene decreased. This is unlike the commonly found abnormal gene in Africa where malaria is still prevalent.