I was reading a textbook (iGenetics, 3rd edition page 71), and came across the following passage:
Homozygous bS bS people make Hb-S, the defective hemoglobin, with two normal a chains specified by wild-type a-globin genes and two abnormal b chains specified by the mutant b-globin bS allele: these people have sickle-cell anemia. Heterozygous bA bS people make both Hb-A and Hb-S and have sickle-cell trait. Because only one type of b chain is found in any one hemoglobin molecule, only two types of hemoglobin molecules are possible—one with two normal b chains, the other with two mutant b chains.
My question is: Why is only one type of beta chain found in any one hemoglobin molecule? Or is this statement simply false?
I did search biology SE for similar questions, and found this one, but it is phrased poorly and the only answer does not address my question.