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 _SPAN_NAVY genes
as well as nonfunctional _SPAN_GREEN pseudogenes.
Humans express different versions of the gene in each family as they develop from embryo to fetus and adult.
These genes evolved from _SIZ_duplication.html duplication
and mutation of an ancestral globin gene.