I recently used one of those services where you send a saliva sample to a lab somewhere for DNA analysis. A part of the study just came back, telling me that I am 99.5% Ashkenazi Jewish, with perhaps a solitary Egyptian (!) ancestor 5 to 8 generations back. (Some tell me, however, that such very low % estimates are not very reliable.)
Mentioning this to friends on Facebook, I found that a whole lot of Jewish people in my age range who grew up in the US Northeast also did tests of this kind, and likewise were repeatedly found to be 98% or more Ashkenazi Jewish.
This suggests a truly striking degree of genetic isolation over many centuries, which of course is now perhaps trending way downwards. It pertains not just to marriage but also to anything else that might have led to children who had children. Given the centuries-long oppression of European Jews, one might have expected more forcible violations of Jews' genetic isolation. But perhaps any such children didn't fare as well for social / cultural reasons.
I also gather that all living Ashkenazi Jews are thought to be the survivors of a population bottleneck, descended from just a few hundred people (at most) who lived in late medieval times. (Were all the other European Jews from the diaspora murdered?) But there is controversy about their Middle Eastern as opposed to European ancestry, with possible gender differences. (Y chromosomes, obviously, pass only through the male line, while mitochondrial DNA passes only through the female line.)
Still waiting for the report on my degree of Neanderthal ancestry. You know the old joke (actually, I think it's mine): "Why am I 98% genetically the same as a chimp, but only 50% the same as my parents, siblings, and children?"