= G-gamma globin
{c}
{title2=$\mathrm G\gamma$}
\b[<G-gamma globin>] is the human <HBG2> <gene> product, a form of <gamma-globin> distinguished by <glycine> at residue 136. The corresponding <A-gamma globin>, encoded by <HBG1>, has <alanine> there. The letter G identifies that <amino acid residue>; it does not indicate a different oxygenation state or a gamma ray.<Human globin gene records in NCBI Gene>[HBG2, Gene 3048, RefSeq Summary: G-gamma/HBG2 versus A-gamma/HBG1]{ref}
Two gamma chains and two <alpha-globin> chains form the standard <fetal hemoglobin> assembly, $\alpha_2\gamma_2$. Identifying a G-gamma chain is a sequence-level result. It does not by itself determine the identity of the other gamma chain in the same <protein complex>.
An original accounting example makes this distinction precise. Suppose 100 hypothetical <fetal hemoglobin> tetramers contain 120 G-gamma and 80 A-gamma chains. Let $x,y,z$ count tetramers with two G-gamma chains, one of each, and two A-gamma chains, respectively. Then
$$
2x+y=120,\qquad y+2z=80,\qquad x+y+z=100.
$$
Nonnegative counts imply $20\le x\le60$, with $y=120-2x$ and $z=x-20$. Thus a G-gamma chain fraction of 0.60 is compatible with 20% to 60% of tetramers containing two G-gamma chains. Assuming independent placement would select $x=36,y=48,z=16$, but independence is additional information, not a consequence of the chain counts.
<Gamma-chain pairing bounds from bulk composition> therefore matter whenever an assay breaks an assembly into its constituent chains before counting them. Sequence abundance, assembly composition and oxygen-binding behavior answer different questions.
The historical distinction emerged from direct chemical examination of <fetal hemoglobin> components. The 1968 primary report described <glycine> and <alanine> at position 136 in material from 17 additional infants, supporting gamma-chain heterogeneity.<Evidence for multiple structural genes for the gamma chain of human fetal hemoglobin by W. A. Schroeder and others>[p.537, opening paragraph]{ref} Modern gene annotations associate the G-gamma form with <HBG2>; the earlier observation was a protein-sequence result rather than direct sequencing of that locus.
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