of course I don't doubt the science per see, but the prelevation, interpretation, and the scope of the ones who make these DNA tests
First off, with DNA there is no interpretation. Either you possess a gene or you don't. That is a fact that cannot be changed and we have known that now for over thirty years. It is why PCR analysis on DNA is sought after in so many criminal cases.
As too the reason that a Jew, or any other minority, in a population looks like the majority is more cultures effect on genes. Standards of beauty generally follow the dictates of the dominant culture. It is why so many women/men around the world follow the American and Western European idea of beauty, and this is reflected partially in who you wish as a mate (doesn't mean you get what you desire though).
When most of the population wants a specific type of beauty it tends to skew the future generations towards that ideal. Of course over time the ideal will change and cause a shift to a slightly different idea.