is a deficiency in the production of one of the globin chains (alpha or beta) that make up hemoglobin.
Normally synthesis of these is balanced. When imbalanced, one globin accumulates relative to the ither forming inclusions that are damaging to the red cells that contain them. The rbcs of a pt with thalassemia are microcytic, but there's other causes of microcytosis. Fe deficiency #1.