Mendelian recessive inheritance describes sickle cell anemia. Human hemoglobin undergoes structural alterations between foetal and adult life. Every human hemoglobin is made up of two distinct pairs of globin chains, which are peptide chains that are joined to the heme, the component that carries oxygen. Adult hemoglobin (hemoglobin A) is made up of α and β chains (α2β2), whereas fetal hemoglobin is made up of two pairs of α and two pairs of γ chains (α2γ2). In sickle cell anemia patients, hemoglobin S is different from hemoglobin A due to a single amino acid change in the β-globin chain: valine instead of glutamic acid. Position 6 is the change in the 146 amino acids that make up the β chains, which are counted starting from the N-terminal end.
One normal β chain gene, βA, and one faulty gene, βS, are present in people with a single sickle cell mutation, or carriers of the aberrant gene. They therefore have two different forms of hemoglobin: sickle cell (α2β2S) and normal (α2β2). We refer to this as the "sickle cell trait." Individuals who receive a βS gene from both parents are only able to produce sickle cell hemoglobin, which results in sickle cell disease.