Sickle cell trait occurs when a person inherits a sickle cell disease gene but does not develop symptoms. Sickle cell disease is a blood disorder in which the body produces an abnormal type of the oxygen-carrying substance hemoglobin in the red blood cells.

Normal hemoglobin is called hemoglobin A, but people with sickle cell disease have only hemoglobin S, which turns normal, round red blood cells into abnormally curved (sickle) shapes.

Normally, a person inherits two genes (one from each parent) that produce beta-globin, a protein needed to produce normal hemoglobin (hemoglobin A). A person with sickle cell trait inherits one normal beta-globin gene (hemoglobin A) and one defective gene (hemoglobin S).

People with sickle cell trait rarely have symptoms due to the condition, because they also have some normal hemoglobin. But they can pass the sickle cell gene to their children.

A person in whom both beta-globin genes are abnormal (they produce hemoglobin S) has sickle cell disease, which can cause serious problems. Both parents must have either the sickle cell trait or the disease itself for a child to have sickle cell disease.