An 18-year-old woman has a low hemoglobin value of 10.5 g/dL on routine CBC testing. The red cells are microcytic, but the WBC and platelet counts are normal. She has an adequate intake of iron-containing foods, her menstrual flow is normal, and there is no other history of blood loss. Her physical examination is completely normal, and iron studies reveal a normal ferritin level. She states that other members in her family have also been told they are anemic and given a diagnosis of alpha-thalassemia. Formation of which of the following hemoglobin's is increased in alpha-thalassemia?