This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': 'Embryonic hemoglobin is: \nSelect and explain. \nA. Alpha 2 Beta 2, B. Alpha 2,Gamma 2, C. Alpha 2 Delta 2, D. Alpha 2', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Normal mammalian hemoglobins contain two pairs of unlike polypeptide chains: one chain of each pair is or -like and the other is non- (, , or ). The -chains of all human hemoglobins encountered after early embryogenesis are the same. The non- chains include the -chain of normal adult hemoglobin (Hb A ), the -chain of fetal hemoglobin (Hb F ), and the -chain of Hb A2 (Hb A2 ), the minor component, which accounts for 2.5 percent of the hemoglobin of normal adults.The -chain of fetal hemoglobin (Hb F) differs from the -chain by 39 residues. Ref: Natarajan K., Townes T.M., Kutlar A. (2010). Chapter 48. Disorders of Hemoglobin Structure: Sickle Cell Anemia and Related Abnormalities. In J.T. Prchal, K. Kaushansky, M.A. Lichtman, T.J. Kipps, U. Seligsohn (Eds), Williams Hematology, 8e.', 'role': 'assistant'}]