The oxygen-carrying hemoglobin comprises two kinds of polypeptides coded by families of genes on different chromosomes.

Chromosome 16 contains the α-globin gene family.

Chromosome 11 contains the β-globin gene family.

Each family contains functional genes as well as nonfunctional pseudogenes.

Humans express different versions of the gene in each family as they develop from embryo to fetus and adult.

These genes evolved from duplication and mutation of an ancestral globin gene.