Normal red blood cells slide easily through narrow blood vessels. In sickle cell disease, many red blood cells change to a crescent shape like a slender moon. Crescent-shaped blood cells cause blockages in blood vessels. These blockages decrease the transport of oxygen. People with sickle cell disease often have sharp pains due to the lack of oxygen in parts of the body. Sickle cell disease is caused by an abnormal type of red blood cell gene inherited by the parents. What is the name for different forms of the same gene?