HBA1

HBA1 is the human gene encoding an alpha-globin chain of hemoglobin. It lies in the alpha-globin region of chromosome 16 alongside HBA2. The reference coding sequences of HBA1 and HBA2 are identical, although parts of their surrounding and untranslated sequences differ.^[ref]

The gene is a particular region of DNA; its product is a protein. Two different loci can therefore produce the same protein sequence. An alpha-globin peptide measurement alone need not reveal how much came from HBA1 rather than HBA2. Alpha chains contribute both to hemoglobin A, with two alpha and two beta-globin chains, and to fetal hemoglobin, with alpha and gamma-globin chains.

An original measurement example makes the ambiguity explicit. Let $a_1,a_2$ denote amounts of identical alpha-chain product contributed by the two loci. A perfectly measured total $a=a_1+a_2=100$ units fits contributions $(40,60)$ or $(20,80)$. More precise measurement of the same total cannot distinguish these possibilities. A locus-sensitive RNA or DNA observation supplies a different kind of evidence.

This also separates gene dosage from final hemoglobin abundance. The number of gene copies constrains what genetic material is present, while gene expression, translation and assembly determine how much functional product is produced. Naming HBA1 identifies the locus being discussed, not a unique alpha-chain molecule that can always be traced back to that locus by its amino-acid sequence alone.