Globin Gene Organization

To understand diseases resulting from genetic alterations in the structure or synthesis of hemoglobin, it is necessary to grasp how the hemoglobin genes, which direct the synthesis of the different globin chains, are structurally organized into gene families and also how they are expressed.
A. α-Gene family
The genes coding for the α-globin and β-globin subunits of the hemoglobin chains occur in two separate gene clusters (or families) located on two different chromosomes (Fig. 1). The α-gene cluster on chromosome 16 contains two genes for the α-globin chains. It also contains the ζ gene that is expressed early in development as an α-globin-like component of embryonic hemoglobin. [Note: Globin gene families also contain globinlike genes that are not expressed, that is, their genetic information is not used to produce globin chains. These are called pseudogenes.]

Figure 3.17 Organization of the globin gene families. Hb = hemoglobin.

B. β-Gene family
A single gene for the β-globin chain is located on chromosome 11 (see Fig. 1). There are an additional four β-globin-like genes: the ε gene (which, like the ζ gene, is expressed early in embryonic development), two γ genes(Gγ and Aγ that are expressed in HbF), and the δ gene that codes for the globin chain found in the minor adult hemoglobin HbA2.