Adult hemoglobin is a protein made of two different types of parts: alpha-like chains and beta-like chains. These parts are controlled by two entirely separate gene clusters located on two different chromosomes:

1. The β-Globin Cluster (Chromosome 11)

This cluster contains five functional genes arranged in the order they are expressed during human development:
HBE1 (ε – epsilon): Used only in the early embryo.
HBG2 (G_γ – gamma): Used during fetal development.
HBG1 (A_γ – gamma): Used during fetal development.
HBD (δ – delta): Makes a minor adult hemoglobin (HbA2).
HBB (β – beta): Makes the major adult hemoglobin (HbA).

2. The α-Globin Cluster (Chromosome 16)

To make working hemoglobin, the body must combine β-cluster proteins with α-cluster proteins.
HBZ (ζ – zeta): Used in the early embryo.
HBA2 (α₂ – alpha 2): Used from fetus through adulthood (about 72% of total alpha-globin).
HBA1 (α₁ – alpha 1): Used from fetus through adulthood (about 28% of total alpha-globin).

This video explains how the various types of hemoglobin are produced from these components (color-coding matches the above text).