Sickle cell trait is highest in West Africa (25% of the population). It also has a high prevalence rate in South and Central Americans, especially in panama. However, it also very infrequently appears in Mediterranean countries such as Italy, Greece and Spain, where it most likely expanded via the selective pressure of malaria, a disease that was endemic to the region. It has been described in Indians, Middle Easterners (such as Arabs and Iranians), Native American people, North Africans and Turks.
Due to the adaptive advantage of the heterozygote, the disease is still prevalent, especially among people with recent ancestry in malaria stricken areas such as Africa, the Mediterranean, India and the Middle East. (Kwiatkowski, 2005).
Malaria was historically endemic to Southern Europe, but it was declared eradicated in the mid-20th century, with the exception of rare sporadic cases (Poncon, et al, 2007). The malaria parasite has a complex life cycle and spends part of it in red blood cells. In a carrier, the presence of malaria parasite causes the red blood cells with defective hemoglobin to rupture prematurely making the plasmodium unable to reproduce. The polymerization of Hb effects the ability of the parasite to digest Hb in the first place. Therefore, in areas where malaria is a problem, people’s chance of survival actually increase if they carry sickle cell trait.
In the USA, where there is no endemic malaria, the prevalence of sickle cell anaemia among blacks is lower (about 0.25%) than in West Africa (about 4.0%) and is falling. Without endemic malaria, the sickle cell mutation is purely disadvantageous and will tend to be selected out of the affected population. (Williams, et al, 2005).
However, the so called African American community of the USA is known to be the result of significant admixture between several African and non-African ethnic groups and also represents the descendants of survivors of the slavery and the slave trade. Thus, a lower degree of endogamy and particularly, abnormally high health selective pressure through slavery may be the most plausible explanations for the lower prevalence of sickle cell anaemia among Afro-Americans compared to sub-saharan African people. Another factor limiting the spread of sickle cell genes in North America is the absence of cultural proclivities to polygamy, which allows affected males to continue to seek unaffected children with multiple partners. (Tishk off and Verell, 2004).