The Hemoglobin molecule is a tetrameric structural protein that comprises two alpha and two beta subunits. The subunits are transcribed from different, genetically linked gene loci. The arrangement comprises two symmetrical alpha + beta subunit halves. Other quaternary structures may comprise a dozen or more polypeptides, transcribed from different gene loci on different chromosomes.

    [The heme groups in Hemoglobin are inorganic Fe⁺⁺ molecules in the dyad tertiary structure of the individual monomers; they are not typical of quaternary structure].