The hemoglobin (Hb) molecule consists of polypeptide chains whose chemical structure is genetically controlled. The normal adult Hb molecule (Hb A) consists of 2 pairs of chains designated as alpha (α) and beta (β). Normal blood also contains a <2.5% concentration of Hb A2 (composed of 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 the place of β chains, and gradually decreases, particularly in the first months of life, until it makes up < 2% of total Hb in adults. The genes for Hb F and Hb A are closely related and exist in the same gene cluster on chromosome 11.
Various structural abnormalities of the hemoglobin molecule can lead to hemoglobinopathies and include the following:
- Amino acid substitution: Sickle cell hemoglobin
- Amino acid deletion: Hb Leiden
- Amino acid addition: Hb Constant Spring
- Chain fusion: Hb Lepore
The majority of hemoglobinopathies result from amino acid substitutions on the beta hemoglobin chain loci.
Globin chain loci are found on chromosome 11 (beta, delta, epsilon, and gamma) and chromosome 16 (alpha and zeta).