 HBG1

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.^[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.