{c}

\b[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.<Human globin gene records in NCBI Gene>[Human HBA1, Gene3039; HBA2, Gene3040; RefSeq summaries]{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.


 HBA1 (source code)