Human Genetic Variation
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Transcript for the Meaning of Genetic Variation Video

Approximately 1 in 625 African Americans in the United States has an inherited disorder called sickle cell disease. This condition results from a change in the structure of their hemoglobin molecules. When the blood contains an abundance of oxygen, this hemoglobin and the red blood cells that contain it appear normal. But when the level of oxygen in the blood drops, the hemoglobin forms crystals that cause the cells to become stiff and sickle shaped. Because sickled cells cannot move easily through the body's tiny capillaries, blood flow is slowed and may become blocked. The sickling also causes the red blood cells to become more fragile and easily damaged. The resulting symptoms can include pain, damage to a variety of organs and tissues, and even death. Sickle cell hemoglobin differs from normal hemoglobin by a single amino acid. Normal hemoglobin has the amino acid glutamic acid at a certain location, whereas, sickle cell hemoglobin has a valine there. This replacement is caused by a mutation in the DNA that codes for the hemoglobin molecule.

The Sickle cell

A person with sickle cell disease has inherited one mutated hemoglobin gene from each parent. Some people, however, inherit only one abnormal gene from either their mother or their father. These people have what scientists call sickle cell trait. Because much of their hemoglobin is normal, they do not experience the symptoms that people with sickle cell disease often do. Scientists have made an interesting observation about the incidence of sickle cell disease in the United States as compared with its incidence in Africa. Only about a quarter of 1 percent of African Americans have the disease. In comparison, as many as 4 percent of Africans have the disease.

Why is the incidence of sickle cell disease higher in Africa than in the United States? People with sickle cell trait are resistant to a deadly form of malaria that occurs in Africa, but not in the Untied States. Thus, in Africa, people with sickle cell trait have a survival advantage over people who do not carry this trait. This advantage means that the proportion of people in Africa with sickle cell trait is higher than it is in the Untied States. And as they reproduce, this leads to an increased number of people with sickle cell disease.

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