Sickle cell disease

Sickle cell disease (SCD) is a group of inherited disorders of the beta-haemoglobin chain. Normal haemoglobin has 3 different types of haemoglobin - haemoglobin A, A2, and F. Haemoglobin S in sickle cell disease contains an abnormal beta globin chain encoded by a substitution of valine for glutamic acid on chromosome 11. This is an autosomal recessive disorder. Sickle cell disease refers to a specific genotype in which a person inherits one copy of the HbS gene and another gene coding for a qualitatively or quantitatively abnormal beta globin chain. Sickle cell anaemia (HbSS) refers to patients who are homozygous for the HbS gene, while heterozygous forms may pair HbS with genes coding for other types of abnormal haemoglobin such as haemoglobin C, an autosomal recessive mutation which substitutes lysine for glutamic acid. In addition, persons can inherit a combination of HbS and beta-thalassaemia. The beta-thalassaemias represent an autosomal recessive disorder with reduced production or absence of beta-globin chains resulting in anaemia. Other genotype pairs include HbSD, HbSO-Arab and HbSE.