= Beta-globin
{title2=$\beta$}
\b[<Beta-globin>] is the human <HBB> gene product that combines with <alpha-globin> in <hemoglobin A>. The standard adult assembly contains two alpha and two beta chains, $\alpha_2\beta_2$.<Human globin gene records in NCBI Gene>[HBB, Gene 3043, nomenclature and Summary]{ref} Beta-globin is one component of that assembly, not another name for the entire hemoglobin molecule.
Primary structural evidence comes from human deoxyhemoglobin record 4HHB, which identifies the alpha and beta chains within its tetramer.<The crystal structure of human deoxyhaemoglobin at 1.74 A resolution by G. Fermi and others>[RCSB 4HHB, Macromolecules and Biological Assembly]{ref} Oxygen binding changes the state of a hemoglobin assembly without changing a beta chain into a gamma chain. Subunit identity and oxygen occupancy are separate descriptions.
An original assembly model illustrates why a protein-mixture measurement need not equal a gene-allele count. Suppose a pool contains two distinguishable beta-chain forms in proportions $p=0.7$ and $1-p=0.3$. If two beta positions were filled independently from this pool, the predicted tetramer fractions would be
$$
P_{11}=p^2=0.49,\qquad
P_{12}=2p(1-p)=0.42,\qquad
P_{22}=(1-p)^2=0.09.
$$
These are conditional predictions of random pairing, not an assertion that real hemoglobin assembly is always independent or that every <heterozygote> makes this mixture. Differential <gene expression>, chain recovery and assembly preferences can change the measured proportions.
The example separates the fraction of a chain form from the fraction of tetramers containing it. In the model, 30 percent of beta chains are form 2, while 51 percent of tetramers contain at least one form-2 chain. A molecular assay should therefore specify whether it reports a sequence, chain abundance, assembled-protein composition or a functional oxygen-binding quantity. Those observations can support one another without being interchangeable.
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