I learned a bit about this when I found out I'm roughly 18% Ashkenazi Jew. One of the reasons Judaism is still an ethnicity despite the Jewish people having migrated to Europe a thousand years ago is because there was a Jewish law against intermarriage outside their religion. Certainly people occasionally married someone who converted, but with a thousand years of fairly strict adherence to that law, the bloodline stayed pretty pure. I took the 23andMe test and found out most Ashkenazi Jews are related to each other, and thus I'm now aware of hundreds of distant cousins through that site. If humans can maintain an ethnicity through the artificial distinctions of belief, I see this as evidence that race itself is a cultural construct. Physiologically, we are all mixable race, with various phenotypes that evolved within different climates for various adaptive purposes.

@PinkPassion -- Are you referring to the reference to markers for Ashkenazi Jew?

i wonder if they can test for hillbilly.

I guess that has to do with the fact that the DNA of Jews, in general, is quite specific, because of the many years of inbreeding within all Jewish communities, because of isolation of the religious groups all over the world. Maybe it's not too much a separate race, but in a way they are. Keeping the Judaic society pure. That means there are quite some DNA-related illnesses that you find specifically in this group.

Judaism is ethnoreligious - an ethnic group with (in most cases) a shared religious background. In other words, it's possible to see if you have any ethnic Jewish ancestry. This also, of course, means it's possible to be religiously Jewish without Jewish ancestry (converts should, under Jewish law, be considered entirely Jewish), or to be non-religious (or even follow another religion entirely) yet still be ethnically Jewish. It gets complicated - Judaism is like that.