In his 2000 State of the Union Address, President Bill Clinton chose to emphasize something he had recently heard from a genome researcher: that humans are all, irrespective of race, 99.9% the same genetically. “Modern science,” he told his country's legislators, “has confirmed what ancient faiths have always taught: the most important fact of life is our common humanity.” Seven years on, and four years after the final publication of the sequences from the Human Genome Project, new technologies and larger data sets are allowing genome biologists to answer a conundrum embodied in that unity-inspiring percentage: if our DNA is...