Sickle cell anemia
occurs when 2 abnormal hemoglobin genes are present and a person's body produces sickle cell hemoglobin exclusively.
1 sickle cell gene and a normal gene produce sickle trait, which has no symptoms. Other abnormal hemoglobin genes can combine with a sickle cell gene and produce other types of anemia. There are several other genetic hemoglobin abnormalities.