Sickle cell anemia is caused by an error in the gene that tells the body how to make hemoglobin. The defective gene tells the body to make the abnormal hemoglobin that results in deformed red blood cells. Children who inherit copies of the defective gene from both parents will have sickle cell anemia. Children who inherit the defective sickle hemoglobin gene from only one parent will not have the disease, but will carry the sickle cell trait. Individuals with sickle cell trait generally have no symptoms, but they can pass the sickle hemoglobin gene on to their children.
The error in the hemoglobin gene results from a genetic mutation that occurred many thousands of years ago in people in parts of Africa, the Mediterranean basin, the Middle East, and India. A deadly form of malaria was very common at that time, and malaria epidemics caused the death of great numbers of people.
Studies show that in areas where malaria was a problem, children who inherited one sickle hemoglobin gene and who, therefore, carried the sickle cell trait - had a survival advantage. Unlike the children who had normal hemoglobin genes, they survived the malaria epidemics they grew up, had their own children, and passed on the gene- for sickle hemoglobin.
As populations migrated, the sickle cell-mutation spread to other Mediterranean areas, further into the Middle East and eventually into the Western Hemisphere. In the United States and other countries where malaria is not a problem, the sickle hemoglobin gene no longer provides a survival advantage. Instead, it may be a serious threat to the carrier's children, who may inherit two abnormal sickle hemoglobin genes and have sickle cell anemia.
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