{c}

\b[HBG2] is the human <gene> encoding <G-gamma globin>, one of the gamma-chain forms contributing to <fetal hemoglobin>. It is a distinct locus from <HBG1> in the beta-like globin region of <chromosome> 11. The curated annotation distinguishes the <HBG2> and <HBG1> products by <glycine> versus <alanine> at globin residue 136.<Human globin gene records in NCBI Gene>[Human HBG2, Gene3048; HBG1, Gene3047; RefSeq summaries]{ref}

The <gene> is not an alternative name for the entire <fetal hemoglobin> tetramer. That complex contains two <alpha-globin> and two <gamma-globin> chains. The G-gamma share within a gamma-chain pool and the size of that pool relative to other globin production are separate composition questions.

An original count example makes the denominators explicit. Suppose a hypothetical production count has $G=30$ <G-gamma globin> chains, $A=70$ <A-gamma globin> chains and $B=900$ <beta-globin> chains. Then
$$
\frac{G}{G+A}=0.30,\qquad
\frac{G+A}{G+A+B}=0.10.
$$
Thirty percent of the gamma pool is G-gamma, while gamma chains make up ten percent of the represented beta-like chain pool. Neither number by itself is a direct measurement of assembled <fetal hemoglobin> fraction, because the example has not specified alpha-chain availability, assembly or degradation.

This is useful when interpreting <gene expression> experiments. A report normalized only within gamma transcripts can change because of <HBG2>, <HBG1>, or both; it does not independently establish the absolute gamma output. Matching the denominator and molecular level prevents a relative expression result from being mistaken for a complete measurement of <hemoglobin> composition.


 HBG2 (source code)