<prompt> Cause of alpha thalassemia:Deletion of alpha genesDeletion of beta genesExcess of alpha genesSingle amino acid substitution in alpha chain  <response>: Ans. a. Deletion of alpha genesCause of alpha thalassemia is deletion of alpha genes.The a-thalassemias are caused by inherited deletions that result in reduced or absent synthesis of a-globin chains.Normally, there are four a-globin genes, and the severity of a-thalassemia depends on how many a-globin genes are affected. As in b-thalassemias, the anemia stems both from a lack of adequate hemoglobin and the effects of excess unpaired non-a. chains b, g, and d, which vary in type at different ages. In newborns with a-thalassemia, excess unpaired g-globin chains form g4 tetramers known as hemoglobin Barts, whereas in older children and adults excess b-glohin chains form b4 tetramers known as HbH. Since free b and g chains are more soluble than free a chains and form fairly stable homo tetramers, hemolysis and ineffective erythropoiesis are less severe than in b-thalassemias. A variety of molecular lesions give rise to a-thalassemia, but gene deletion is the most common cause of reduced a -chain synthesis. - Robbins 9e p64Ia-THALASSEMIASClinical SyndromesGenotypeClinical FeaturesMolecular GeneticsSilent carrier-a aaAsymptomatic; no red cell abnormalityMainly gene deletions0a-Thalassemia trait-- aa AsianAsymptomatic, like b-thalassemia minor -a -a black African, Asian HbH disease-- -aSevere; resembles b-thaiassemia intermedia Hydrops fetalis-- --Lethal in utero without transfusions 