“From a technology perspective we can test anything,” said Mark Hughes, director of the Genesis Genetics Institute in Detroit, who is performing P.G.D. this month for two couples who want to avoid passing on a susceptibility to breast cancer. “The issue becomes what is considered serious enough to warrant such testing and who decides that.”

The process is also difficult and expensive. P.G.D., which requires in vitro fertilization, can cost tens of thousands of dollars. While insurance companies often pay for the more traditional uses of the procedure, they have not done so for cancer-risk genes, fertility experts say. The barrier to affordability, some critics fear, could make preimplantation diagnosis for cancer risk the first significant step toward a genetic class divide in which the wealthy will become more genetically pure than the poor.

Knowing that Mr. Kingsbury had tested positive for the colon cancer mutation, the Kingsburys started with the basic laws of genetics: because children randomly inherit half of each parent’s genes, he had a 50 percent chance of passing it on. Since the mutation raises the risk of developing the cancer by about twentyfold, that means any child of theirs conceived the traditional way would have about a one in three chance of getting it, usually around age 45. Those who did develop the cancer would also have a nearly 90 percent chance of surviving it, but only if it was caught early.

The jumble of odds meant little to the Kingsburys as they tried to think about starting a family while the cancer claimed Mr. Kingsbury’s second uncle. Then, from a cousin of Mr. Kingsbury’s who had also fought colon cancer, they heard about P.G.D., the technology that offered them a way to reload the genetic dice.

To do it, they had to overcome their own misgivings about meddling with nature. They had to listen to the religious concerns of Mr. Kingsbury’s family and to the insistence of Ms. Kingsbury’s that the expense and physical demands of in vitro fertilization were not worth it, given that the couple could probably get pregnant without it. They had to stop asking themselves the unanswerable question of whether a cure would be found by the time their child grew up.

It took them two months to make the decision. But every time Mr. Kingsbury looks at Chloe, with her blue saucer eyes and her tantrums that turn abruptly to laughter — and back — he knows it was worth it.

“I couldn’t imagine them telling me my daughter has cancer,” he said, “when I could have stopped it.”