What is the spatial/cultural "resolution" of DNA testing for genealogical purposes? How finely can it be used to estimate where a particular person's ancestors were from? I am interested in testing the hypothesis that some of my wife's ancestors who came to the US from the Ukraine had previously emigrated from Germany to the Ukraine. My hypothesis is based on their last names (which are more German-sounding than Russian or Ukrainan), but I am wondering if it would be possible to refute or corroborate it using DNA evidence.
So the basic question is this: does the DNA of Jews from the Ukraine differ from the DNA of Jews living in Germany, and can this difference be detected reliably?