ALPHA THALASSEMIA
Alpha thalassemia is the result of deficient or absent synthesis of alpha globin chains, leading to excess beta globin chains. Alpha globin chain production is controlled by two genes on each chromosome 16 (Table 14,5). Deficient production is usually caused by a deletion of one or more of these genes. A single gene deletion results in alpha thalassemia silent carrier status, which is asymptomatic with normal hematologic findings. The two-gene deletion causes alpha thalassemia trait (minor) with microcytosis and usually no anemia. The three-gene deletion results in significant production of hemoglobin H (HbH), which has four beta chains (beta4). Alpha thalassemia intermedia, or HbH disease, causes microcytic anemia, hemolysis, and splenomegaly. The four-gene deletion results in significant production of hemoglobin Bart's (Hb Bart's), which has four gamma chains (gamma4). Alpha thalassemia major with Hb Bart's usually results in fatal hydrops fettles.