People with thalassemia have an inherited blood disorder that causes mild or severe anemia (uh-NEE-me-uh). The anemia is due to reduced hemoglobin (he-mo-GLOBE-in) and fewer red blood cells than normal. Hemoglobin is the protein in red blood cells that carries oxygen to all parts of the body. The two main types of thalassemia, alpha and beta, are named for the two protein chains that make up normal hemoglobin. The genes for each type of thalassemia are passed from parents to their children. There are mild and severe forms of the disease, the latter often called Cooley's anemia. Cooley's anemia is the most common severe form of thalassemia seen in the U.S. Alpha thalassemia occurs when one or more of the four genes needed for making the alpha globin chain of hemoglobin are variant or missing. Moderate to severe anemia results when more than two genes are affected. Alpha thalassemia major can result in miscarriages. Beta thalassemia occurs when one or both of the two genes needed for making the beta globin chain of hemoglobin are variant. The severity of illness depends on whether one or both genes are affected, and the nature of the abnormality. If both genes are affected, anemia can range from moderate to severe.