= Gamma-globin
{title2=$\gamma$}
\b[<Gamma-globin>] is the globin-chain class that pairs with <alpha-globin> in <fetal hemoglobin>, whose standard composition is $\alpha_2\gamma_2$. In humans, the <HBG1> and <HBG2> genes encode closely related gamma-chain products.<Human globin gene records in NCBI Gene>[HBG1/HBG2, Genes 3047 and 3048, Summary]{ref} The NCBI annotations distinguish A-gamma from G-gamma by alanine versus glycine at residue 136; gamma is therefore not necessarily a single sequence label.
The human fetal deoxyhemoglobin structure 1FDH provides a molecular example, identifying alpha and gamma entities within the tetramer.<Structure of human foetal deoxyhaemoglobin by J. A. Frier and M. F. Perutz>[RCSB 1FDH, Macromolecules and Biological Assembly]{ref} The gamma chain occupies a protein-subunit role; it is not a gamma photon or a description of the molecule's oxygenation state.
An original amount calculation makes the subunit distinction useful. Suppose an assay quantifies 120 pmol total gamma chains, with 90 pmol of one sequence form and 30 pmol of the other. Their chain fractions are 0.75 and 0.25. If every measured gamma chain belongs to fetal tetramers containing exactly two gamma chains, total fetal hemoglobin is
$$
n_{\mathrm{HbF}}=\frac{120}{2}=60\,\mathrm{pmol}.
$$
If 12 pmol of that measured gamma pool is instead free or belongs to other material, the same tetramer interpretation gives $(120-12)/2=54$ pmol. A total chain assay does not by itself prove where every chain was assembled.
A separate sequence-selective assay can establish which gamma forms are present, while a composition or functional measurement tests the assembled hemoglobin. Changes in <HBG1> or <HBG2> expression are questions about the producing system; changes in <oxyhemoglobin> versus <deoxyhemoglobin> are questions about oxygen binding. Keeping those levels separate prevents a molecular identity result from being mistaken for a complete physiological measurement.
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