Historians have long debated whether Thomas Jefferson had a sexual relationship with one of his slaves, Sally Hemings. Now we have an answer. Retired pathologist Eugene Foster collected blood samples from 14 men, black and white, who claim to be descended from Jefferson. The distinctive, largely unchanging Y (male) chromosomes of Jefferson's white descendants, Foster writes in this week's Nature, almost precisely match those of descendants of Hemings' last son, Eston, who was born at Monticello in 1808. They do not match those of her first son, long thought to be Jefferson's, or those of Jefferson's two nephews, who were often said to have sired Hemings' children.