<P> Most types of normal hemoglobin, including hemoglobin A, hemoglobin A2, as well as hemoglobin F, are tetramers composed of four protein subunits and four heme prosthetic groups . Whereas adult hemoglobin is composed of two α (alpha) and two β (beta) subunits, fetal hemoglobin is composed of two α subunits and two γ (gamma) subunits, and is commonly denoted as α γ . Because of its presence in fetal hemoglobin, the γ subunit is commonly called the "fetal" hemoglobin subunit . </P> <P> In humans, the gamma subunit is encoded on chromosome 11, as is the beta subunit . There are two similar copies of the gamma subunit gene: γG which has a glycine at position 136, and γA which has an alanine . The gene that codes for the alpha subunit is located on chromosome 16 and is also present in duplicate . </P> <P> When fetal hemoglobin production is switched off after birth, normal children begin producing adult hemoglobin (HbA). Children with sickle - cell disease instead begin producing a defective form of hemoglobin called hemoglobin S which aggregates together and forms filaments that cause red blood cells to change their shape from round to sickle - shaped . These defective red blood cells have a greater tendency to stack on top of one another and block blood vessels . These invariably lead to so - called painful vaso - occlusive episodes, which are a hallmark of the disease . </P> <P> If fetal hemoglobin remains the predominant form of hemoglobin after birth, the number of painful episodes decreases in patients with sickle - cell disease . Hydroxyurea promotes the production of fetal hemoglobin and can thus be used to treat sickle - cell disease . The fetal hemoglobin's reduction in the severity of the disease comes from its ability to inhibit the formation of hemoglobin aggregates within red blood cells which also contain hemoglobin S. Combination therapy with hydroxyurea and recombinant erythropoietin--rather than treatment with hydroxyurea alone--has been shown to further elevate hemoglobin F levels and to promote the development of HbF - containing F - cells . </P>

How does the dissociation curve for fetal hemoglobin compared to that for maternal hemoglobin