Current proposed model for the evolution of α- and β-globin clusters from paralogous clusters in different lineages. The unlinked α- and β-globin clusters in birds and mammals evolved from an ancient in trans duplication of the ancestral linked α-β cluster, followed by differential gene silencing (marked with X). This resulted in bird β-like globin genes (β2) orthologous to the marsupial ω-globin gene (β2 beside the α-globin cluster) but paralogous to mammalian β-like globin genes (β1). Adapted from Wheeler et al. .