As someone working on converting to Judaism, I can tell you that the situation is complicated, with ethnicity and religion strongly interrelated. Anyone that claims Jewishness solely involves religion is just as wrong as anyone that claims Jewishness solely involves ethnicity. You cannot think about Judaism the same way you'd think about, say, Christianity.

With that disclaimer in mind, the vast majority of Jews - due to traditional prohibitions against intermarriage and low numbers of converts - originate from the general area where the State of Israel is today. Here, both the founding myths in the Torah and modern genetic testing are in agreement.