The clinical severity of a-thalassemia relates to the
number of genes affected out of the four a genes and correlates with the presence of the g4 or b4
tetramers, and the extent to which they reduce erythropoiesis and red cell survival[11]. Recently, a
correlation between a/b-globin mRNA ratio and disease severity was shown[12](Table 1). The most
serious and frequently fatal of the a-thalassemia syndromes is that of the four gene–deletion syndrome,
hydrops fetalis[13]. Deletion of three a genes, resulting in hemoglobin H (Hb H) disease, has a mild to
moderately severe phenotype, while deletion of two or one a-globin genes has no clinical significance