 HBA2

HBA2 is a human gene encoding alpha-globin, located with HBA1 on chromosome 16. The two genes share the same reference coding sequence, but their 3′ untranslated regions differ substantially in the curated annotation.^[ref] Identical encoded chains therefore coexist with distinguishable genomic and transcript contexts.

A usual diploid reference configuration contains two copies of each locus: one HBA1 and one HBA2 on each homologous chromosome. This gives four alpha-chain gene copies in that configuration. Copy number is a gene dosage description, not a claim that all four copies contribute equal amounts of RNA or protein.

An original linear measurement model illustrates how a distinctive transcript region could add information. Suppose a validated common-region response gives $y_1=m_1+m_2$, while an independently validated HBA2-specific response gives $y_2=m_2$. With synthetic responses $y_1=100$ and $y_2=60$ in compatible units, the model yields $m_2=60$ and $m_1=40$. Without the second response, only the total is identified.

This model does not provide an assay design or assert perfect specificity for any actual reagent. It isolates the experimental requirement: the additional observation must distinguish the loci rather than repeat their shared sequence signal. Measurements targeting different 3′ untranslated regions can answer a different question from measurements of the identical alpha-globin product. The identity of the gene, the amount of its transcript and the assembled hemoglobin remain separate levels of evidence.