Sickle cell disease is an inherited structural disorder of haemoglobin. The disorder specifically involves the P globin subunit of the haemoglobin molecule such that instead of normal adult haemoglobin Hb A (a2P2), Hb S results (a2PS2) (Fig. 1.1). Sickle cell disease is recognized to occur in particular ethnic groups, notably individuals of African, Asian or more rarely Mediterranean ancestry in whom the variant allele is present at a relatively high frequency. It is one of the most common inherited diseases known in man, with an estimated incidence in African Americans of 1 in 625 and a carrier frequency of 8%; carrier frequencies of up to 20% have been reported in Uganda and Kenya (Young 2005).
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