This Smithsonian Magazine article explores the power of genetics to reveal information about one's ancestry, so-called "genetic geneology." The article tells the story of several Hispanic families who were revealed to have Jewish ancestry from Spain (in the 15th century, Spanish Jews were forced to either convert to Catholicism or flee the country). The link was made when women from these families were found to carry a genetic mutation that causes breast and ovarian cancer. This particular mutation, known as 185delAG, is primarily found in women of Jewish descent.
If you are interested in learning more about the genetics of ancestry, Dr. Rick Kittles (University of Chicago) gave a 2007 lecture on this topic for the Illinois Humanities Council (view YouTube video: part 1 | part 2 | part 3).