Old-fashioned gene hunting wasn’t terribly efficient. Geneticists typically pursued one gene at a time, armed only with guesses—usually wrong—about which chunks of genetic code might be linked to human disease. Geneticists managed to bag a few trophies anyway—genes for Huntington’s chorea and cystic fibrosis, for example—mostly in rare diseases caused by a problem in a single, high-powered gene. Unfortunately, most of the more common diseases, such as type II diabetes, are instead controlled by a whole crowd of gene variants, each playing a small and often subtle rol...