Normally, a person inherits two genes that tell the body to
hemoglobin A. One gene comes from each parent.
People who inherit one defective hemoglobin S gene and one normal
hemoglobin A gene have
sickle cell trait. These people don't have symptoms of
sickle cell disease nor do their bodies make sickled blood cells. But they can
pass the defective hemoglobin S gene to their children.
Sickle cell disease occurs when a person inherits one defective
hemoglobin S gene from each parent.
sickle cell disorders occur when a person inherits a
hemoglobin S gene from one parent and another type of defective hemoglobin gene
from the other parent. Sickle cell disease is a sickle cell disorder.