What causes Alpha thalassemia?

"The mutations that cause thalassemia cause problem to the normal production of hemoglobin and cause low hemoglobin levels and red blood cell destruction, causing anemia.  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 fetalis. When you're anemic, your blood doesn't have enough red blood cells to carry oxygen to your tissues — leaving you fatigued."