Sickle Cell Disease - Risk Factors

Risk Factors

Sickle cell disease is inherited. People at risk for inheriting the gene for sickle celldescend from people who are or were natives of Africa and parts of India and the Mediterranean. The sickle cell gene also occurs in people from South and Central America, the Caribbean, and the Middle East. The high incidence of the sickle cell gene in these regions of the world is due to the sickle cell's ability to make red blood cells resistant to the malaria parasite:

• People who inherit just a single gene are referred to as having the sickle trait. These people are protected against malaria and do not develop sickle cell disease. About 40% of people in certain parts of Africa and about 9% of African Americans have the trait.
• Those who inherit both copies of the HbS gene develop sickle cell disease. They are not protected from malaria, however. In fact, malaria is more serious in these individuals. An estimated one in every 500 African Americans and one in every 1,000 to 1,400 Hispanic Americans are born with sickle cell disease itself.

Risk in Children of Parents with the Sickle Cell Gene

The sickle cell gene for hemoglobin S (HbS) is the most common inherited blood condition in America. About 72,000 Americans -- mostly African Americans -- have sickle cell disease. The risk for inheriting sickle cell disease from parents with the sickle cell gene is as follows:

• One parent has only one copy of the sickle cell gene and the other parent has two normal hemoglobin genes, and the child inherits a healthy gene from each parent. The child will not inherit either the disease or the trait.
• The child inherits one copy of the sickle cell gene. The child has the trait (HbS) only. The other, healthy hemoglobin gene overrides HbS and blocks the development of sickle cell disease. Such people lead normal lives.
• The child inherits the hemoglobin S gene from both parents (HbSS). The child develops the full-blown disease. (If each parent has one copy of the gene, the child has a 25% chance of acquiring the disease.)
• The child inherits one hemoglobin S gene and one abnormal hemoglobin gene from other causes (such as one form called HbSC). Such children may develop a form of sickle cell disease. It is often a milder variant but children can experience severe symptoms. They are also at risk for some of the complications of sickle cell disease, although their risks for serious problems are lower than in children with the full-blown disease.