What causes Beta thalassemia?

"Beta thalassemia is the result of deficient or absent synthesis of beta globin chains, leading to excess alpha chains. 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. When you're anemic, your blood doesn't have enough red blood cells to carry oxygen to your tissues — leaving you fatigued."