The hemoglobin (Hb) molecule consists of polypeptide chains whose chemical structure is genetically controlled. The normal adult Hb molecule (Hb A) consists of two pairs of chains designated as alpha (α) and beta (β). Normal blood also contains a ≤3% concentration of Hb A2 (composed of αlpha (α) and delta (δ) chains). Hemoglobin F (Hb F) is predominantly hemoglobin that is present during fetal development. Hb F has gamma (γ) chains in place of β chains and gradually decreases, particularly in the first months of life, until it makes up <1% of total Hb in adults.
Normal adult hemoglobin:
- 95–98% hemoglobin A
- 1–3% hemoglobin A2
- <1% hemoglobin F
The genes for Hb F and Hb A are closely related and exist in the same gene cluster on chromosome 11. We inherit one gene from each parent for the production of our globin chains on chromosome 11. Hence, we each have two separate genes encoding the production of epsilon, beta, gamma, and delta chains (the globin chain genes controlled by chromosome 11). The majority of hemoglobinopathies result from amino acid substitutions on the beta hemoglobin chain loci.
Globin chain loci are also found on chromosome 16 and control alpha and zeta chain production. Since two genes on chromosome 16 are responsible for alpha chain production and one gene for zeta chain production, each parent contributes two genes for alpha chain production and one gene for zeta chain production. In total, we have four genes regulating the production of alpha chains and two genes regulating the production of zeta chains.
