Since scientists first mapped the human genome more than a decade ago, excitement has built about a new era of personalized medicine, in which people will ultimately be able to use the information embedded in their DNA to better prevent and treat disease.

But even as the technologies that decode genomes have gotten faster and cheaper, questions have remained about how useful that information will be to a doctor and a patient trying to understand risk for a complicated disease caused by a combination of genetic and environmental factors, such as diabetes or cancer.