What Increases Your Risk

Risk of inheriting sickle cell disease

Sickle cell disease is an inherited blood disorder, passed from parent to child. Children with sickle cell disease have two defective genes, one from each parent. Various forms of sickle cell disorder occur when a person inherits one sickle cell gene and one other type of defective hemoglobin gene.

People who inherit one defective hemoglobin S gene and one normal hemoglobin A gene have sickle cell trait. They don't have symptoms of sickle cell disease nor do their bodies make sickled blood cells. But they have a 50% chance of passing the defective hemoglobin S gene to each of their children.

• If both parents have sickle cell trait, each of their children will have a 1-out-of-4 (25%) chance of having sickle cell disease.
• If one parent has sickle cell disease (has two genes for making hemoglobin S) and the other has sickle cell trait (has one hemoglobin S gene and one normal hemoglobin A gene), each of their children will have a 1-out-of-2 (50%) chance of having sickle cell disease and a 1-out-of-2 (50%) chance of having sickle cell trait.
• If one parent has sickle cell disease (two hemoglobin S genes) and the other has two normal hemoglobin A genes, each of their children will have sickle cell trait. None of the children will have sickle cell disease.

People whose ancestors were from Africa, India, the Middle East, the Mediterranean (Turkey, Italy, Greece), and some Latin American countries are more likely to inherit the gene that can cause sickle cell disease. In the United States, about 2,000 children are born each year with sickle cell disease.²

For more information, see a picture of the risk of passing on an autosomal recessive disease such as sickle cell disease.

Risk of painful events and complications caused by sickling

Triggers that can cause red blood cells to sickle include:

• Lack of oxygen because of:
  • Cigarette smoke. Both smoking and inhaling smoke from other people's cigarettes (secondhand smoke) can damage the lungs and lower oxygen levels in the blood. Children and adults who have sickle cell disease should avoid cigarette smoke.
  • High altitude [locations higher than 5000 ft (1524 m)]. There is less oxygen in the air at high altitudes.
  • Air travel. Because there is less oxygen at high altitudes, flying in an unpressurized airplane can cause cells to sickle. Large passenger airplanes are pressurized. But a person with sickle cell disease may have a sickling problem even when flying in a pressurized airplane. Most people won't have problems if they are flying only for a short time (less than 4 to 6 hours) and drink plenty of fluids during the flight.
• Cold temperatures and wind. Exposure to cold air, wind, and water may cause a painful event by triggering red blood cell sickling in exposed areas of the body.
• Loss of fluid (dehydration). Dehydration may slow blood flow in the body, which increases the chance of having a painful event.
• Infection. Painful events are commonly triggered by infection.
• Stress. Increased stress may bring on painful events.
• Strenuous exercise. Dehydration and reduced oxygen levels in a person's blood resulting from strenuous exercise may cause red blood cells to sickle. Many people with sickle cell disease can tolerate moderate exercise if they rest when they feel tired and drink plenty of fluids.