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What Causes Sickle Cell Anemia?

Sickle cell anemia is an inherited disease. People who have the disease inherit two genes for sickle hemoglobin—one from each parent.

Sickle hemoglobin causes red blood cells to develop a sickle, or crescent, shape. Sickle cells are stiff and sticky. They tend to block blood flow in the blood vessels of the limbs and organs. Blocked blood flow can cause pain and organ damage. It can also raise the risk for infection.

Sickle Cell Trait

People who inherit a sickle hemoglobin gene from one parent and a normal gene from the other parent have sickle cell trait. Their bodies make both sickle hemoglobin and normal hemoglobin.

People who have sickle cell trait usually have few, if any, symptoms and lead normal lives. However, some people may have medical complications.

People who have sickle cell trait can pass the sickle hemoglobin gene to their children. The following image shows an example of an inheritance pattern for sickle cell trait.

Example of an Inheritance Pattern for Sickle Cell Trait

The image shows how sickle hemoglobin genes are inherited. A person inherits two hemoglobin genes—one from each parent. A normal gene will make normal hemoglobin (A). A sickle hemoglobin gene will make abnormal hemoglobin (S).

The image shows how sickle hemoglobin genes are inherited. A person inherits two hemoglobin genes—one from each parent. A normal gene will make normal hemoglobin (A). A sickle hemoglobin gene will make abnormal hemoglobin (S).

When both parents have a normal gene and an abnormal gene, each child has a 25 percent chance of inheriting two normal genes; a 50 percent chance of inheriting one normal gene and one abnormal gene; and a 25 percent chance of inheriting two abnormal genes.

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September 28, 2012 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.

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