HBG1 (source code)
= HBG1
{c}
\b[HBG1] is the human <gene> encoding the <A-gamma globin> form of <gamma-globin>. It lies with <HBG2> in the beta-like globin region of <chromosome> 11. Two gamma chains together with two <alpha-globin> chains form <fetal hemoglobin>.<Human globin gene records in NCBI Gene>[Human HBG1, Gene3047, RefSeq summary and product names]{ref}
The curated summary distinguishes <A-gamma globin> from <G-gamma globin> at residue 136 in its globin numbering convention: <alanine> is associated with the <HBG1> product and <glycine> with the <HBG2> product. The A/G labels describe amino-acid forms, not an instruction to read a particular <DNA> base from the gene symbol. A sequence comparison must also state its protein-numbering convention.
The two genes illustrate why total signal and source-specific expression are different measurements. In an original example, let gamma transcript amounts be $g_1=70$ and $g_2=30$ units. A measurement sensitive only to the shared gamma signal gives $g_1+g_2=100$; it cannot by itself distinguish that mixture from $(g_1,g_2)=(30,70)$. Evidence from a distinguishing sequence feature supplies an independent constraint.
Likewise, a relative <HBG1> fraction within the gamma pool is not the same quantity as the amount of all <fetal hemoglobin> in a sample. Other globin chains, total synthesis and assembly also matter. The <gene> identifies a source of one gamma-chain form; the <RNA>, chain and tetramer measurements answer related but different biological questions.
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