Figures from the paper below (additional labeling by Razib).
A genome-wide genetic signature of Jewish ancestry perfectly separates individuals with and without full Jewish ancestry in a large random sample of European Americans
Anna C Need, Dalia Kasperaviciute, Elizabeth T Cirulli and David B Goldstein
Genome Biology 2009, 10:R7doi:10.1186/gb-2009-10-1-r7
Conclusions: Here we show that within Americans of European ancestry there is a perfect genetic corollary of Jewish ancestry which, in pinciple, would permit near perfect genetic inference of Ashkenazi Jewish ancestry. In fact, even subjects with a single Jewish grandparent can be statistically distinguished from those without Jewish ancestry. We also found that subjects with Jewish ancestry were slightly more heterozygous than the subjects with no Jewish ancestry, suggesting that the genetic distinction between Jews and non-Jews may be more attributable to Near-Eastern origin for Jewish popuations than to population bottlenecks.