any of a group of disorders caused by the presence of variant hemoglobin in the red blood cells. Variant-hemoglobin disorders occur geographically throughout the Old World in a beltlike area roughly the same as that of malaria. The presence of variant hemoglobin in moderate amounts may constitute a selective advantage in that it provides some protection from the lethal effects of malaria, thereby allowing more persons to reach reproductive age. The most important of the hemoglobinopathies are sickle-cell anemia (sickle cell anemia) and thalassemia. Hemoglobin C (Hb C) is relatively common among African blacks living north of the Niger River and is found in 2–3 percent of blacks in the United States. Hemoglobin C disease (occurring when the variant Hb C gene is inherited from both parents) produces such symptoms and signs as vague pain, jaundice, enlarged spleen, mild to moderate anemia, and some hemorrhaging. The life span of the individual, however, is normal, and the disease is much milder than the sickle-cell anemia found in the same geographic range. It is possible that Hb C is gradually replacing Hb S (variant hemoglobin of sickle-cell anemia) by a process of selection in Africa; Hb C does not produce early mortality in homozygotes (persons with two genes for Hb C), as does sickle-cell anemia, yet Hb C may afford some protection from malaria.

Hemoglobin D is found mainly in people of Afghan, Pakistani, and northwestern Indian descent, but it also occurs in those of European ancestry. Hemoglobin D disease (two genes for Hb D) may produce mild hemolytic anemia. Hemoglobin E is widespread in Southeast Asia, being found especially among Thai, Cambodian, Laotian, Malaysian, Indonesian, Vietnamese, and Burmese peoples. Hemoglobin E disease (two genes for Hb E) may result in a mild microcytic (small red blood cell) anemia. Hemoglobin E–thalassemia disease (one gene for Hb E, one gene for thalassemia) is severe and clinically closely resembles thalassemia major. Hemoglobin H, found in many groups in the Old World (e.g., Chinese, Thai, Malayans, Greeks, Italians), has almost always been identified in combination with thalassemia; symptoms resemble those of thalassemia.

Many other variant hemoglobins—such as types G, J, K, L, N, O, P, Q, and variants of Hb A (“normal” hemoglobin) and Hb F (fetal hemoglobin)—are known but typically do not produce clinical manifestations.