“There will be more common variants to find,” Dr. Hunter said. “It would be unfortunate if we gave up now.”

Dr. Goldstein, however, said it was “beyond the grasp of the genomewide association studies” to find rare variants with small effects, even by recruiting enormous numbers of patients. He said resources should be switched away from these highly expensive studies, which in his view have now done their job.

“If you ask what is the fastest way for us to make progress in genetics that is clinically helpful,” he said, “I am absolutely certain it is to marshal our resources to interrogate full genomes, not in fine-tuning our analyses of common variations.”

He advocates decoding the full DNA of carefully selected patients.

Dr. Kraft and Dr. Hunter say that a person’s genetic risk of common diseases can be estimated only roughly at present but that estimates will improve as more variants are found. But that means any risk estimate offered by personal genomics companies today is unstable, Dr. Kraft said, and subject to upward or downward revision in the future.

Further, people who obtain a genomic risk profile are likely to focus with horror on the disease for which they are told they are at highest risk. Yet this is almost certain to be an overestimate, Dr. Kraft said.

The reason is that the many risk estimates derived from a person’s genomic data will include some that are too high and some that are too low. So any estimate of high risk is likely to be too high. The phenomenon is called the “winner’s curse,” by analogy to auctions in which the true value of an item is probably the average of all bids; the winner by definition has bid higher than that, and so has overpaid.

Dr. Kari Stefansson, chief executive of deCODE Genetics, an Icelandic gene-hunting company that also offers a personal genome testing service, said deCODE alerted clients to pay attention to diseases for which testing shows their risk is three times as great as average, not to trivial increases in risk.