A new paper in PLoS ONE, Evaluation of Group Genetic Ancestry of Populations from Philadelphia and Dakar in the Context of Sex-Biased Admixture in the Americas, doesn't add much to what we know. They looked at a several hundred individuals who are self-identified as African American and European American, as well as 49 Senegalese from Dakar. Additionally, they reanalyzed data from Latin America from whites and blacks in Brazil, as well as a group of mixed Cubans. They found what you might expect to find, African and Native ancestry shows a female bias, European ancestry shows a male bias.
But Figure 3 is nice in that it illustrates how exceptional European Americans are: they are a New World population with very little admixture, that is, a "pure race." The data on people of white identification in most of Latin America, including in the European dominated southern cone, is that a non-trivial load of non-European genes exists in these populations (often indigenous ancestry down the maternal lineage). Similarly, African origin populations in the New World are invariably known to be admixed, more or less. Finally, even the "indigenous" generally show non-trivial European or African ancestry (this is evident in the regular notations of African or European ancestry being used to explain anomalous results).*
Citation: Stefflova K, Dulik MC, Pai AA, Walker AH, Zeigler-Johnson CM, et al. 2009 Evaluation of Group Genetic Ancestry of Populations from Philadelphia and Dakar in the Context of Sex-Biased Admixture in the Americas. PLoS ONE 4(11): e7842. doi:10.1371/journal.pone.0007842
* Some escaped African slaves "went native" and assimilated into indigenous tribes. The symbiotic relationship between the Seminoles of Florida and escaped blacks is well known, but not exceptional. The case in the United States is exceptional in its magnitude in that most people of native self-identifications are mixed.
White Americans from Philadelphia are unlikely to have any Native or African ancestry. Most are descended primarily from 19th or early 20th century Irish, Polish, Italian and Jewish immigrants.
Of course the amount of Native ancestry in Euro-Americans has been exaggerated by family legends, and the actual amount, where it exists, is often going to be undetectable by commercially available DNA tests.
If one is looking for such ancestry, it is best to look at French Canadians, as well as old-stock Americans with Scots and Scots-Irish roots, especially in the South and West. Scots were heavily involved in the colonial-era southeastern deerskin trade (likewise the later Canadian fur trade) and many of the early mountain men were of Scots-Irish descent.
The Euro-American results are false.
No Negroid mtDNA was found in the Euro-American sample. The only African mtDNA is two North African Caucasoid lineages. These are most likely from Southern Europeans.
None of the "Native American/Asian" lineages are Amerind.
Not surprising that a small proportion of North African mtDNA might be found in Euro-Americans from Philadelphia, given the number of Philadelphians of Southern Italian/Sicilian descent.
Among Euro-Americans in general, there is clearly less non-European ancestry than among the most "European" populations in Latin America (Argentines) or even among white Afrikaners (many of whom have minor African or South/Southeast Asian ancestry). But if one is looking for such ancestry among Euro-Americans, the sample used in this case is about the LAST place one would want to look.