Ancestral proportions and admixture dynamics in geographically defined African Americans living in South Carolina

Abstract

We analyzed admixture in samples of six different African-American populations from South Carolina: Gullah-speaking Sea Islanders in coastal South Carolina, residents of four counties in the "Low Country" (Berkeley, Charleston, Colleton, and Dorchester), and persons living in the city of Columbia, located in central South Carolina. We used a battery of highly informative autosomal, mtDNA, and Y-chromosome markers. Two of the autosomal markers (FY and AT3) are linked and lie 22 cM apart on chromosome 1. The results of this study indicate, in accordance with previous historical, cultural, and anthropological evidence, a very low level of European admixture in the Gullah Sea Islanders (m = 3.5 +/- 0.9%). The proportion of European admixture is higher in the Low Country (m ranging between 9. 9 +/- 1.8% and 14.0 +/- 1.9%), and is highest in Columbia (m = 17.7 +/- 3.1%). A sex-biased European gene flow and a small Native American contribution to the African-American gene pool are also evident in these data. We studied the pattern of pairwise allelic associations between the FY locus and the nine other autosomal markers in our samples. In the combined sample from the Low Country (N = 548), a high level of linkage disequilibrium was observed between the linked markers, FY and AT3. Additionally, significant associations were also detected between FY and 4 of the 8 unlinked markers, suggesting the existence of significant genetic structure in this population. A continuous gene flow model of admixture could explain the observed pattern of genetic structure. A test conditioning on the overall admixture of each individual showed association of ancestry between the two linked markers (FY and AT3), but not between any of the unlinked markers, as theory predicts. Thus, even in the presence of genetic structure due to continuous gene flow or some other factor, it is possible to differentiate associations due to linkage from spurious associations due to genetic structure.

Admixture analysis of South Asian cattle

“…The alleles in the category III histogram that occur at 20% or more in the European population and have an SFD value greater than 40% are likely to be ancestry informative and have been utilised in the following analyses (Figure 2). An SFD value of 40% excludes few taurine alleles and similar levels of frequency differential have been found to be useful in other studies (McKeigue, 1998;Parra et al, 2001).…”

The Genetic Diversity of the Americas

“…In Brazil, for instance, the highest levels of European ancestry have been documented in the south of the country (the area of strongest European immigration during the "whitening" of Brazil), whereas the highest levels of African ancestry are in the northeast (historically the main area of African slave concentration) (72,82). Similarly, in US African Americans, African ancestry is highest in southern states, the area historically most strongly associated with slavery (9,19), and a south-to-north gradient of increasing European admixture has been interpreted as related to the northern migration of African Americans after the abolition of slavery (9,19,68,69).…”

“…Finally, several communities tracing their ancestry to marooned slaves have been prime objects of study in multidisciplinary analyses examining the origin in Africa of the slaves who were transported to America (68,84,96). Genetic studies have shown that, although many of these populations have mainly African ancestry (8,68), some have substantial non-African admixture, indicating that contrary to what was previously thought, maroon settlements were not always that isolated (84).…”

Gender bias in the multiethnic genetic composition of central Argentina

“…The relatively low density of Native Americans coupled with the resistance of these population groups to Spanish acculturation and slavery led to the introduction of very large numbers of sub-Saharan Africans to the region. Although the demographic impact of Africans on the autochthonous population in other Latin American countries such as Colombia and Brazil was extremely high and still persists in modern populations (Alves-Silva et al 2000;Beleza et al 2005;Ely et al 2006;Parra et al 2001;Salas et al 2005b, c;, their real demographic impact in Argentina is a debatable topic. Some scholars maintain that a very significant number of African slaves were forcedly moved to Argentina, in particular to regions of intense agricultural activity: according to Victoria-Gomes (2002), in 1778 the population of Africans in Córdoba was *44%, whereas in 1887 the official percentage of Africans was reduced to *1.8%.…”