In a-thalassemia with -

a -thalassemia is charecterised by the reduced or absent synthesis of a- globin chain.a-Thalassemia Unlike b-thalassemia, a-thalassemia is caused mainly by deletions involving one or more of the a-globin genes. The severity of the disease is propoional to the number of a-globin genes that are missing (Table 11-3). For example, the loss of a single a-globin gene produces a silent- carrier state, whereas the deletion of all four a-globin genes is lethal in utero because the red cells have viually no oxygen-delivering capacity. With loss of three a-globin genes there is a relative excess of b-globin or (early in life) g-globin chains. Excess b-globin and g-globin chains form relatively stable b4 and g4 tetramers known as HbH and Hb Ba, respectively, which cause less membrane damage than the free a-globin chains that are found in b-thalassemia; as a result, ineffective erythropoiesis is less pronounced in a-thalassemia. Unfounately, both HbH and Hb Ba have an abnormally high affinity for oxygen, which renders them ineffective at delivering oxygen to the tissues Referance : Exam preparatory manual for UGs by Ramdas Nayak Page no:273 Ref Robbins 9/e pg415