Sickle Cell Anemia

Genetics and Incidence of Sickle Cell Anemia

The genetics of sickle cell anemia are not complicated. Sickle cell anemia is a beta chain variant and inheritance of the beta chains is located on chromosome 11. Chromosome 11 has one location on each chromosome for the inheritance of a normal beta chain or an abnormal beta chain; therefore, the sickle cell anemia is autosomal codominant, inherited in simple Mendelian fashion (Fig. 8.1). At present, there are 80,000 Americans who have sickle cell disorders spread among 65% with sickle cell disease, 24% with hemoglobin SC disease, and 10% with sickle cell beta thalassemia.4 Individuals born with sickle cell trait were not included in the percentages. African American babies born with sickle cell disease occur with a frequency of1:375. The sickle gene is especially prominent in African populations near areas endemic for malaria including Central and West Africa, some parts of the Mediterranean, Asia, and India. The sickle gene is seen frequently in African American populations and with increasing frequency in nonblack populations.5 The presentation of symptoms in individuals with sickle cell anemia is highly variable, a direct result of the different haplotypes of hemoglobin S that are inherited. Each haplotype differs from the other by possessing different sequences of some nucleotides in the DNA strands, but they are all located in the same gene cluster. There are four primary haplo-types of the sickle beta gene: Asian, Senegal, Benin, and Bantu.6 Haplotypes may be inherited homzygouosly or heterozygously. Each of these haplotypes differs in the amount of hemoglobin F the red cell possesses. Higher hemoglobin F concentrations mean a less severe clinical presentation. The Asian haplotype is seen in

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