I recently had to answer a question on Quora as to why many Jewish people show up as having gentile DNA:

“How could Ashkenazi-Jews as ethnic Jews be genetically 40% Southern-European mainly Southern-Italian and Greek if Jews mostly stuck to themselves?”

Unfortunately, much of that could have come from rape. Some of it came from Sefardim, many of whom themselves picked it up from rape. I myself got a nasty surprise when one of the tests on Gedmatch put me in a population match with “Russian Cossack“—get enough of us whose ancestors were raped by gentiles together, and we may not or will likely not be getting any autosomal Jewish DNA overtime.

Also, some Jewish autosomal DNA is in fact misclassified as gentile DNA – partly because people who submit the DNA to DNA reference panels either do not know that they are Jewish or in fact lie about their heritage for multiple reasons (including Antisemitism which they face in their current locations—including Eastern Europe and Russia, and they often either do not know or will not tell that ancestors were pogrom or Holocaust survivors).

There’s also the fact that AncestryDNA has admitted to throwing out DNA samples that do not pass what they consider their standards—and throwing out something in order to achieve selection or confirmation bias is never acceptable. Additionally, many of the reference panels are in fact skewed in number—often not having 400 people at minimum per group, and often not having 1:1 ratios. If I knew that I was going to be studying research methods in college just to see what we learned totally discarded (and I learned that the minimum acceptable sample is actually 1000), who knows if I would’ve even gone to college? I was majoring in Political Science, and the research methods apply across both soft and hard sciences—including political science and biological science (e.g., genetics).

Finally among other reasons, for multiple reasons, DNA testing is actually illegal in Israel—including the excuse about “mamzerut” (and anyone who has done previous DNA testing often has to access sites such as Ancestry through VPNs). This of course does not help those of us in the Diaspora whom are trying to prove that we are Jewish in order to make aliyah.