Hb switching at the human α- and β-globin loci. Embryonic globins are shown in green, fetal globins in blue, and adult globins in red. Primitive erythropoiesis, derived from the yolk sac, is characterized by expression of ζ-globin (from the α-globin locus) and ε-globin (from the β-globin locus). These are silenced at ∼8 weeks of gestation. α-globin then accounts for the entirety of the transcriptional output from the α-globin locus. At the β-globin locus, there is a switch to a fetal globin (γ-globin) during fetal life and then a second switch to the adult β-globin. The predominant type of Hb corresponding to each developmental stage is shown below.