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 . In
Northwest Africa the bS
gene frequency can be as high as ~40%.
The compound bS/b-thalassemia
genotype is frequent in Mediterranean countries. 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. Approximately 8% of the
African-American population is reported to carry the bS-globin
gene. Here in Canada, SCD affects approximately 1 in 20,000 Canadians.
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).
four most common types of SCD states are: Hb SS(bS/bS),
and Hb S/b-thal0
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. Together Hb S/b-thalassemias
have a combined estimated incidence of 1:1667 live births.