Sickle cell disorders are characterized by the presence of sickle hemoglobin (Hb S) in red blood cells. Hb S is a structural disorder caused by valine replacing glutamic acid in the sixth position on the beta chain.* Sickle cell disorders are inherited disorders. If an individual inherits a sickle gene (S) from one parent and one normal gene (A) from the other parent, the disorder is a heterozygous condition known as sickle cell trait (Hb SA). If an individual inherits "S" genes from both parents, the disorder is a homozygous condition known as sickle cell anemia (Hb SS). Sickle cell anemia can also demonstrate hereditary persistence of fetal hemoglobin (Hb S/HPFH).
A double heterozygous condition known as Hemoglobin SC disease also exists where one beta chain carries the mutation for Hb S and the other beta chain carries the mutation for Hb C. In addition, Hb S can be present with thalassemia (also called Mediterranean anemia and is the result of improper hemoglobin formation).
Other Hb S combinations are very rare and include Hb S/Hb E, Hb S/Hb D Los Angeles, Hb S/Hb G-Philadelphia, and Hb S/Hb O Arab.
*The normal amino acid sequence for the beta hemoglobin chain includes glutamic acid in position 6, phenylalanine in position 85, and leucine in position 88. These amino acid positions are key to the sickling process.