I mistakenly assumed that my DNA ethnic breakdown would exactly match my siblings. I also assumed it would proportionally match my ancestry. Genes are far more complex and random than that. For example, my grandfather is of 1/4 Swiss ancestry. Yet, his DNA might not show 25% Swiss genes nor mine 6.25% although it could. I found this out via this article, and the concept is further explained in this article.
This fascinating study of a small sampling of people attempts to analyze the backgrounds of the three main ethnic groups in the U.S.: European Americans, Hispanic Americans, and African Americans. Now, there is no way of knowing if this is a representative sampling, as they note, but I think it is still great for general information. Be sure to look at all the maps. This is something to regularly refer back to.
And in a similar vein, this map displays subgroups and migration patterns and typical generation length in U.S. This matches with my family’s genealogy and some of David Hackett Fischer’s explanations. We’ve always moved West, quite literally.
And if you are ever in the market for DNA testing, this is a thorough analysis of the pros and cons. I’d like to test a couple people in my family for a variety of these tests. The ethnicity one is interesting, but the Y-DNA is probably most helpful for genealogical research.