I like this lesson of Rabbi Mizrachi

[q_q_]

Actually i think if most Europeans go back a hundred of generations many are of or partly Jewish

 I doubt it is a very big number. The Askenazi Jewish community was very small when it came to Europe. True some assimilated (mostly through forced conversion), but it is a very small percentage.

I have heard from someone that read in a book about genetics, that it would take VERY FEW converts/intermarriages over the many centuries to account for all the vast genetic differences and apparent physical feature differences we see today between Ashkenazi Jews and other groups of Jews (Sefaradi etc).   

It seems from what I have read that the 2 populations (Jew and Non Jew) in Europe did NOT mix very much, due to vast cultural differences, different beliefs and of course anti-Jewish racism that persisted from beginning to today.

Reminds me of a classic Kahane talk radio moment.. The host says

There is so much variation amongst jews from different countries, it's because of all the jewish girls sleeping around, isn't it?

Rabbi Kahane responds saying .  Well, if that is the case, then I put to you, what does that say about non-jewish girls?