Several people have asked me this question over the years. Here's a good answer from Ask Moses:
Is there a reliable DNA test for the Jewish race?
The Short Answer:
No, because Jews are not a race.
The Askmoses Answer:
Even a 100% accurate DNA familial match won't necessarily clarify someone's Jewish identity. For example, some people are 'Jewish DNA' positive but not Jewish; because for someone to be born Jewish, his or her mother must be Jewish. So a person can have three Jewish grandparents (and subsequently 'Jewish DNA') yet still not be Jewish, if the maternal grandmother wasn't.
On the other hand some Jewish people are 'Jewish DNA' negative, yet Jewish due to conversion. They were born of a completely non Jewish genetic background and later chose to become Jewish - a spiritual transformation that has not been proven to alter DNA in any manner.
So a genetic screening may indicate biological relation to Jewish people, or the absence of this. It cannot, however, prove whether a person is Jewish or not.1
- 1. Many members of the Lemba tribe in Africa have the 'Kohen gene' common among Jewish Kohens that is passed from father to son. However, as explained, this gene cannot be proof of Jewishness. Geneticists may have used this to conjecture about a Jewish and Kohen male ancestor of the tribe hundreds of years ago.