EXCERPT

There's never been data available on as many people's genes as there is today. And that wealth of information is allowing researchers to guess at any person's chance of getting common diseases like diabetes, arthritis, clogged arteries, and depression.

Doctors already test for rare, deadly mutations in individual genes. Think of the BRCA breast cancer gene. Or the one-letter mutation that causes sickle-cell anemia. But such one-to-one connections between a mutation and a disease – "the gene for X" – aren't seen in most common ailments. Instead, these have complex causes, which until recently have remained elusive.