SICKLE CELL
The frequency of sickle cell trait (Hemoglobin S) in Southern European populations is often used to quantify black ancestry allegedly acquired via large-scale absorption of slaves in recent history. But the evidence below indicates ancient, indirect transmission of HbS to Europe, and explains how environmental selection expands the gene, rendering it useless in tracing ancestral relationships.
Resistence to Malaria
"Since sickle cell anemia is only advantageous in regions where incidence of malaria is high, it occurs less often in populations rarely exposed to malaria. For millions of years, whenever malaria swept through an area of Africa, those with sickle cell trait had a greater chance of survival than those without the trait. Those with sickle cell trait perpetuated the disease. Even today, in some areas of Africa like Uganda, up to 46% of the population may have sickle cell trait, and 2% have full-blown sickle cell anemia. In areas without malaria, sickle cell anemia and sickle cell trait are disadvantageous because of their debilitating side effects. Those with the disease cannot compete against those without the disease, so reproduce less often. Therefore, the S hemoglobin gene is passed on less frequently in these populations, and the incidence of sickle cell anemia and sickle cell trait decreases. For instance, in the United States, where malaria is not a problem, only 8% of the African-American population has sickle cell trait, and 0.25% have full blown sickle cell anemia. Sickle cell anemia and even sickle cell trait are enough of a disadvantage in malaria-free parts of the world that their frequency of incidence decreases."
Not a Reliable Marker
"African admixture in Sicily has been long suspected because of the presence of the sickle gene. Nevertheless, the degree of African admixture cannot be derived from the study of HbS frequency, since this gene was most likely expanded by the selective pressure of malaria, for a long time endemic to the region. We have examined 142 individuals from the Sicilian town of Butera (12% sickle trait) to search for other markers of the globin gene cluster less likely to be selected for by malaria. The TaqI polymorphism in the intervening sequences between the two gamma genes is informative. We have found only two instances of this African marker (TaqI(-)) among 267 normal chromosomes, demonstrating that the admixture occurred at a much lower level than previously thought."
Predates Historical Events
"Throughout this whole area...a manifestation of disease, once called 'osteoporosis symmetrica' occurs in ancient skeletons. It is a porotic hyperostosis of the skulls and other bones.... In the Mediterranean, porotic hyperostosis probably represents thalassemia or sicklemia...."From this standpoint it is important to note that in Greece all the adult frequencies of hyperostosis change thus: Mesolithic 40%, Early Neolithic 60%, Late Neolithic-Early Bronze Age 28%, Middle Bronze 13%, Royal Graves 8%, Late Bronze 8%, Early Iron 4%, Classic 1%, Hellenistic 10%, Roman 24%, Medieval 12%, Turkish 45%, Romantic 37%.
"But a steady decrease in thalassemia from first farmers to the 4th c. BC would fit the probabilities expected from increasingly efficient use of farmland and swamp-drainage.... The subsequent rather quick increase would fit the evidence amassed by Jones (1907, 1909) and Stephanos (1884), and their successors, showing that malaria became endemic and then fluctuated somewhat in severity down to modern times."
Arrived from North Africa
"We conclude that the beta S gene was introduced to Sicily from North Africa and that the gene flow originated in Central West Africa and traveled north through historically well-defined trans-Saharan commercial routes."
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"The consensus is that the gene was introduced into Sicily and Southern Italy from Northern Africa through the trans-Saharan trade routes, or, alternatively, by means of the Greek colonisation...."
Conclusion
"The sickle-cell trait cannot be used for that purpose [a tracer of African ancestry]. Even the assumption that the occasional sickle-cell trait carrier found in Italy and Greece must have inherited the gene from an African forebear can no longer be maintained."
Related Topics
Population Genetics: Tracing ancestry with the use of more reliable markers.
Italians: Actual sub-Saharan DNA in Italy is negligible.
Portuguese: Actual sub-Saharan DNA in Portugal is negligible.
Greeks: Actual sub-Saharan DNA in Greece is negligible.
