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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.

(Emphasis mine)

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.

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For one specific hemoglobin molecule, yes it's just one type of beta chain. In the case of a heterozygote, this hemoglobin molecule could be expressing from the bS gene OR the normal beta gene. Let's call it a 50/50 chance for each hemoglobin to be expressing bS. Because RBCs have a ton of hemoglobin in them, this 50/50 or so expression of bS wouldn't be enough to cause RBC sickling and disease, and it also helps explain the heterozygote advantage vs malaria.

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    $\begingroup$ "Let's call it a 50/50 chance for each hemoglobin to be expressing bS." What? Hemoglobin is a protein, not a cell , it doesn't "express" anything. Each hemoglobin molecule has two beta chains: the question is: why do we never find one normal and one bS chain in the same hemoglobin tetramer. $\endgroup$
    – user59201
    Commented Apr 19, 2020 at 23:46

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