I'm not sure it would be particularly relevant. The only Jewish people who are important are those who self-identify as Jews and who participate in and perpetuate Jewish culture. Without those people, Judaism would cease to exist, even if lawyers named Sidney Finkelstein with curly hair, bald spots, and big noses, who had no knowledge of Judaism, remained.
And religious and cultural Jews are easy to pick out. I don't think any DNA testing will be needed.
No--that doesn't scare me. You know what scares me?
It's fairly obvious that the United States as a nation has fallen very far from God: much farther, in some cases, than other nations which God has been recorded as judging very harshly. Why has God's judgment been withheld in our case? Well, I don't pretend to know the mind of God, but he did promise to bless those who bless Israel--and, of course, to curse those who curse her. The US is right now the best friend Israel or the Jewish people have ever had. Perhaps that has something to do with it.
However, probably in large part because of that, the US is now the only remaining major concentration of Jews outside Israel. Prophecy foretells that the Jewish people as a whole will return to Ha-Aretz, or the Land. (Not necessarily every single last one of them, but the vast majority.) That's got to mean there will be a large Jewish exodus from the US to Israel at some point in the future.
Why would that be, do you think? Because life in Israel, in a tiny little country with the weapons of the entire Arab world trained on it, will somehow become more attractive than life in the US currently is? That'd be nice, but somehow I don't think so.
Somehow I think what's going to chase the Jews out of the US is widespread, rampant anti-Semitism. It's much more realistic to think that the US will at some point in the future go from being a nation that blesses Israel to a nation that curses Israel.
And that's what scares me.