Were I to submit my cells to DNA tests, results would doubtless show that —barring any surprises—I am a Jew. I have four Jewish grandparents. Genetically speaking, I share a common ancestry with most Jews, no matter where they live, what they look like, or how they practice. Thanks to generations of insularity and historically low rates of intermarriage, Ashkenazi Jews (80 percent of all Jews) are one of the most coherent genetic groups that exist; they are “a gold mine” for people who like to study heredity, says Jon Entine, who in 2007 wrote Abraham’s Children, an exploration of Jewishness and DNA. “Jews are more homogeneous than Icelanders or Basques or Costa Ricans,” he says.