Ans. is 'b' i.e., No a-chain Alpha - Thalassemia o People who do not produce enough alpha globin chain have alpha - thalassemia. o Alpha globin chain is made by four genes, each gene contributes to 25% of the a-globin chains. o The severity of a-thalassemia varies greatly depending on the number of a-globin genes affected : - (i) Silent carrier state o Single a-globin gene is deleted. o These individuals are completely asymptomatic (ii) a-Thalassemia trait o Two a-globin genes are deleted. o These individuals are asymptomatic with some red cell abnormalities like 13-thalassemia minor. (iii) HbH disease o Three genes of a-globin chain are deleted. o With only one gene, the synthesis of a-chain is markedly reduced and tetramers of excess 0-globin, called HbH, form. o HbH has extremely high affinity for oxygen and therefore is not useful for oxygen exchange, leading to tissue hypoxia dispropoionate to the level of hemoglobin. o Patients have moderate to severe anemia that may require occasional blood transfusion. (iv) Hydrops fetalis o Deletion of all four a-globin genes. o In the fetus, excess It-globin chains form tetramers, known as hemoglobin Bas. o Hemoglobin Ba has such a high affinity for oxygen that it delivers almost no oxygen to tissues. o Most individuals die before or sholy after bih. o In utero blood transfusion have allowed the bih of children with hydrops fetalis who then require lifelong blood transfusion.