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Prevalence 

Geographically, SCD has a widespread distribution, although it typically affects those with ancestry from areas where the disease is prevalent. The sickle (bS) mutation is most prevalent in parts of Africa, South East Asia (India), the Middle East and Mediterranean. SCD is particularly endemic to parts of the African continent; 120,000 African infants are born with SCD annually [22]. In Northwest Africa the bS gene frequency can be as high as ~40%[22].  The compound bS/b-thalassemia genotype is frequent in Mediterranean countries[17]. However, cases of SCD occur in Europe and also in North America. Based on U.S. statistics, this disease affects more than 50,000 North Americans[23]. Approximately 8% of the African-American population is reported to carry the bS-globin gene[24]. Here in Canada, SCD affects approximately 1 in 20,000 Canadians[25].

Due to the heritable and recessive nature of the bS mutation, two parents who are  HbAS carriers have a 50% probability of having  a child who is also a HbAS carrier, but a 25% probability of having a child with Hb SS. Thus, the heterozygote "carrier" state HbAS (bA/bS) is most prevalent, although, again due to the recessive nature of the disease, these individuals exhibit sickle trait but not SCD (discussed below).  

Incidence

The four most common types of SCD states are: Hb SS(bS/bS), Hb SC(bS/bC), Hb S/b-thal+(bS/bthal+) and Hb S/b-thal0 (bS/bthal0). In the U.S., Hb SS(bS/bS) has an estimated incidence of 1:625 live births, while  the next most common is Hb SC(bS/bC), which has an estimated incidence of 1:835 live births[24]. Together Hb S/b-thalassemias (bS/bthal+/0) have a combined estimated incidence of 1:1667 live births[24].