Because both HbA and HbF have a-chains, genetic disorders of a-chain synthesis result in defective fetal and adult haemoglobin production. In the fetus, a deficiency of a-chains leads to the production of excess y-chains, which form y4-tetramers, or
Hb Bart's; in adults, excess of P-chains form P4-tetramers or HbH. 4
There are two a-globin genes on chromosome 16 (Figure 6.1) and hence there are two main types of a-thalassaemia: a0-thalassaemia, in which both genes are inactivated (-/aa), and a+-thalassaemia, in which only one of the pair is affected (-a/aa). There are two important clinical disorders caused by a-thalassaemia, the Hb Bart's hydrops syndrome and HbH disease. The former results from the homozygous inheritance of a0-thalassaemia (-/-). HbH disease usually results from the co-inheritance of both a0 and a+-thalassaemia (-a/-). These interactions are summarized in Figure 6.10.
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