Globin gene expression and hemoglobin composition during development

Hemoglobin (Hb) consists of a heterotetramer of two alpha-like chains and two beta-like chains. The identity of the chains varies during development, creating different hemoglobins over the course of embryonic and fetal development and after birth.

• Alpha-like chains (shown in blue) include embryonic zeta (ζ) chains, produced during early embyrogenesis, followed by alpha (α) chains.
• Beta-like chains (shown in red) include embryonic epsilon (ε) chains, followed by gamma (γ) chains during fetal development through early infancy. Beta (β) chain production starts during fetal development but does not predominate until after birth. Delta (δ) globin is another beta-like chain that is produced after birth that pairs with beta globin to make Hb A2.

Globin gene expression varies during development, with embryonic Hbs produced during embryonic development, fetal Hb (Hb F) expressed later in gestation through early infancy, and adult Hbs (Hb A and a small amount of Hb A2) expressed in later infancy through adulthood.