European Admixture in African Americans

Y Chromosome STR Haplotypes and the Genetic Structure of U.S. Populations of African, European, and Hispanic Ancestry

Genome Research Vol. 13, Issue 4, 624-634, April 2003

Manfred Kayser et al.,

Abstract To investigate geographic structure within U.S. ethnic populations, we analyzed 1705 haplotypes on the basis of 9 short tandem repeat (STR) loci on the Y-chromosome from 9-11 groups each of African-Americans, European-Americans, and Hispanics. There were no significant differences in the distribution of Y-STR haplotypes among African-American groups, whereas European-American and Hispanic groups did exhibit significant geographic heterogeneity. However, the significant heterogeneity resulted from one sample; removal of that sample in each case eliminated the significant heterogeneity. Multidimensional scaling analysis of RST values indicated that African-American groups formed a distinct cluster, whereas there was some intermingling of European-American and Hispanic groups. MtDNA data exist for many of these same groups; estimates of the European-American genetic contribution to the African-American gene pool were 27.5%-33.6% for the Y-STR haplotypes and 9%-15.4% for the mtDNA types. The lack of significant geographic heterogeneity among Y-STR and mtDNA haplotypes in U.S ethnic groups means that forensic DNA databases do not need to be constructed for separate geographic regions of the U.S. Moreover, absence of significant geographic heterogeneity for these two loci means that regional variation in disease susceptibility within ethnic groups is more likely to reflect cultural/environmental factors, rather than any underlying genetic heterogeneity.

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Sufficient information does exist, however, to permit estimates of the European-American genetic contribution to African-Americans. Previous studies based on nuclear loci have generally found ~20% European genetic contribution to African-American populations (Reed 1969; Chakraborty et al. 1992; Parra et al. 1998; Destro-Bisol et al. 1999; Collins-Schramm et al. 2002), in agreement with our estimate (averaged for mtDNA and the Y-chromosome) of 18%-24%. Our results indicate substantially higher contribution of European-American Y-chromosome (27.5%-33.6%) than mtDNA (9.0%-15.4%) to African-Americans, also in agreement with previous studies (Parra et al. 1998, 2001). Presumably, this disparity in admixture estimates for the Y-chromosome versus mtDNA reflects the greater genetic contribution of European-American men than women to African-Americans during the slavery period. However, there is currently an increasing trend toward more marriages between African-American men and European-American women; census data indicate that in 1960 there were 25,000 marriages involving African-American men and European-American women and 26,000 marriages involving African-American women and European-American men, whereas in 1992, there were 163,000 marriages involving African-American men and European-American women and 83,000 marriages involving African-American women and European-American men (source, U.S. Census Bureau, http://www.census.gov/population/socdemo/race/interractab1.txt). In our study, on the basis of self-reported ancestry, the offspring of marriages between African-Americans and European-Americans would generally be assigned as African-Americans rather than European-Americans. Hence, if this trend continues, the disparity between mtDNA and Y-chromosome-based estimates of the European genetic contribution to African-Americans may eventually diminish or even reverse direction.

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[Interesting]:

In contrast to the African-American groups, the European-American and Hispanic groups do show significant geographic heterogeneity. However, in both cases, this is due to the influence of one group, as removal of that one group reduces the heterogeneity in the remaining groups to statistically insignificant levels. For both European-Americans and Hispanics, it is the Texas group that accounts for the significant geographic heterogeneity. Why this is the case is not obvious; among European-American groups, the Texas group has a low amount of haplotype diversity (but not the lowest) and the lowest MPSD (Table 1), suggesting possibly a lower amount of genetic variation for the Y-chromosome for this group. However, among Hispanic groups, the Texas group does not stand out in terms of either haplotype diversity or MPSD, although this group is quite differentiated in the MDS plot (Fig. 2). Moreover, mtDNA analyses of these same samples do not indicate any differences between these groups and other European-American and Hispanic groups, respectively (Melton et al. 2001). The most likely explanation would appear to be that the significant heterogeneity attributable to the European-American and Hispanic Texans reflects chance rather than any true biological differences; analyses of additional samples from Texas would be required to test this hypothesis.

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