Unlike asking "Are Jews a family?", as historians have traditionally done, geneticists seeking to advise Ashkenazic families are also, in passing, asking, "Do Jews all share the same versions of one or more genes?" -- a question with a testable, precise answer. As no two people except pairs of identical twins have exactly the same version of the human genomic text, this claim could be confirmed or rejected by a search for versions of the human genome shared by all Jews and no other people.
Given the historical context of the Nazi "experiment," it is all the more remarkable that Jews all over the world have been flocking to the new technology of DNA-based diagnosis, eager to lend their individual genomes -- each a surviving data point from the terrible experiment in negative selection -- to a revisiting of this issue of biological Judaism.
At a recent meeting of the Association of Orthodox Jewish scientists and the Columbia Center for the Study of Science and Religion, it became clear that Jewish curiosity has provided sufficient genetic material to give a perfectly clear negative answer: There is no support in the genomes of today's Jews for the calumnious and calamitous model of biological Judaism. Though there are many deleterious versions of genes shared within the Ashkenazic community, there are no DNA sequences common to all Jews and absent from all non-Jews. There is nothing in the human genome that makes or diagnoses a person as a Jew.
If there is no clear DNA marker that makes one a Jew in this century, what DNA markers should we expect Lehi and Nephi to have brought to the New World?
The issue of DNA and the Book of Mormon involves quite a number of interesting issues. One useful new resource in this area is "DNA and the Book of Mormon" by David Stewart, M.D. at Limhi.com. I have also written an extensive article on the topic, with many references that you can check yourself. My article is "Does DNA Evidence Refute the Book of Mormon?" I was pleased when LDS.org chose to place a PDF version of my DNA article on their Website in their newsroom section, along with some articles from FARMS.
Also see the lastest FARMS Review of Books at farms.byu.edu for further information.