We are at a turning point in medicine. Knowledge of the individual's genetic makeup will soon allow molecular medicine to reach deep inside each of us to cure most of the maladies that afflict us—and perhaps even slow the rate at which we age. First we will learn to understand each person's genome; then we will learn to craft treatments tailored to his or her genetic constitution.

But it may not be so easy—and not for purely scientific reasons. Consider 23andMe, a commercial enterprise launched in 2006 that was merely looking...