The results to the left are from 23andMe for someone whose paternal grandparents were immigrants from southern Germany. Their mother had a father who was of English American background (his father was a Yankee American with an English surname and his mother was an immigrant from England), and grandparents who were German (Rhinelander) and French Canadian respectively on their maternal side.
Looking at the results from 23andMe one has to wonder, why is this individual only a bit under 25% French & German, when genealogical records show places of birth that indicates they should be 75% French & German (more precisely, 62.5% German and 12.5% French). Though their ancestry is 25% English, only 13% of their ancestry is listed as such.
First, notice that nearly half of their ancestry is “Broadly Northwestern European.” Last I checked 23andMe uses phased haplotypes to detect segments of ancestry. This is a very powerful method and is often quite good at zeroing in on people of European ancestry. But with Americans of predominant, but mixed, Northern European background rather than giving back precise proportions often you obtain results of the form of “Broadly…” because presumably, recombination has generated novel haplotypes in white Americans.
But this isn’t the whole story. Why, for example, are many of the Finnish people I know on 23andMe assigned as >90% Finnish, while a Danish friend is 40% Scandinavian?
The issue here is that to be “Finnish” and “Scandinavian” are not equivalent units in terms of population genetics. Finns are a relatively homogeneous ethnic group who seem to have undergone a recent population bottleneck. In contrast, Scandinavia encompasses several different, albeit related, ethnicities which are geographically widely distributed.
Ethnic identities are socially and historically constructed. Additionally, they are often clear and distinct. This is not always the case for population genetic classifications. On a continental scale, racial classification is trivial, and feasible with only a modest number of genetic markers. Why? Because the demographic and evolutionary history of Melanesians and West Africans, to give two concrete examples, are distinct over tens of thousands of years. Population genetic analyses which attempt to identify or differentiate these groups have a lot of raw material to work with.