Sequencing a person's DNA is now a routine task. That reality has left doctors looking for ways to put the technology to work. A decade ago, a top federal scientist said, "Whether you like it or not, a complete sequencing of newborns is not far away." Dr. Francis Collins, who made that statement, has been head of the National Institutes of Health for the intervening decade. But his prophecy hasn't come to pass, for both scientific and practical reasons. Scientists have found that, so far, a complete genetic readout would be a poor substitute for the traditional blood test that...