Important excerpts:
The African-American genetic contribution to European-Americans is below the limits of detection with these methods.
(In other words, practically non-existent. I suppose this may be different with other DNA studies because of the specific isolation of European-Americans, rather than the more ambiguous "white" group. The latter may include individuals of recent non-white admixture yet still call themselves white and non-Europeans who may have more admixture.)
European-American genetic contribution to African-Americans (Table 5) was much higher for the Y-chromosome than for mtDNA; ~27.5%-33.6% of African-American Y-chromosomes were determined to be of European-American ancestry versus only 9.0%-15.4% of African-American mtDNAs.
We found no significant heterogeneity among regional groups of African-Americans, which seems somewhat surprising for two reasons. First, a large number of different African source populations contributed to present-day African-American groups, with about half coming from the area extending from Senegal to Western Nigeria, and the remaining half coming from the area extending from Eastern Nigeria to Angola (Curtin 1969; Reed 1969). However, the amount of genetic heterogeneity among these West and Central African source populations that contributed to African-Americans is not known, as a comprehensive study of genetic variation in these populations has not been carried out.
Second, the amount of admixture of African-Americans with European-Americans is thought to have varied across different geographic regions of the U.S.,
with generally higher levels of admixture observed in Northern groups (Reed 1969; Chakraborty et al. 1992).
A further complicating factor is migration among geographic regions within the United States. Even with heterogeneity in the founding West African populations and/or the subsequent amount of European-American genetic contribution to African-Americans, migration of African-Americans within the United States may have been extensive enough to eliminate between-group differences in Y-STR haplotype frequencies. In particular, during and following World War I, an estimated one million African-Americans (~10% of the African-American population) left rural areas in the southern United States for metropolitan areas in the north (Johnson and Campbell 1981; Tanner 1995). The lack of geographic heterogeneity observed in African-American mtDNA and Y-chromosome types may thus reflect this "Great Migration", the largest internal migration in the history of North America.
The overall lack of geographic heterogeneity among European-Americans is not surprising, as European populations exhibit little differentiation with respect to Y-STR haplotypes (Roewer et al. 2001) and mtDNA types (Melton et al. 1997b). However, the striking uniformity among regional groups of Hispanics for both Y-STR haplotypes and mtDNA types (see Fig. 4) is surprising, given that Hispanic does not refer to a defined geographic region, in contrast to European-American and African-American. Instead, the ethnic category, Hispanic, typically can refer to someone of Mexican, Puerto Rican, Cuban, Central/South American, or other Spanish culture ancestry (Chakraborty et al. 1999), and previous analyses have estimated varying degrees of native American, Spanish, and African ancestry in Hispanic populations (Hanis et al. 1991; Merriwether et al. 1997; Chakraborty et al. 1999).
For the mtDNA SSO-types, the Hispanic and European-American groups were completely separated from one another (Fig. 4), whereas for the Y-STR haplotypes, there was some intermingling of Hispanic and European-American groups.
It appears that the paternal and maternal structure of Hispanic groups differ, most likely reflecting a greater contribution of European-American Y-chromosomes than mtDNA haplotypes to the Hispanic gene pool. Although insufficient data from potential source populations among native North American, Central American, and Caribbean populations exist to permit estimates of admixture for Hispanic groups on the basis of Y-STR haplotypes or mtDNA SSO-types, other studies have found a greater contribution of native American mtDNA than nuclear genes to Hispanic populations (Merriwether et al. 1997), which supports our results indicating a greater contribution of European-American males than females to the Hispanic gene pool.
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/soc...terractab1.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.
(This also doesn't count the majority of sexual unions with black men and white women out of wedlock (70% of black men father children out of wedlock). Thus the number of mulatto children by white women is far greater than estimated.)
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