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.


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 Apr 19 '20 at 23:46

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