The hemoglobin C trait is the presence of an abnormal type of the oxygen-carrying substance in red blood cells (hemoglobin) called hemoglobin C. People with hemoglobin C trait also have some normal hemoglobin (hemoglobin A) so they rarely have problems from the condition, but they can pass on the trait to their children.

A person who has hemoglobin C with little or no normal hemoglobin has hemoglobin C disease. Hemoglobin C disease produces a mild type of anemia that occurs because the red blood cells containing abnormal hemoglobin break down prematurely.