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As Chad Kingsbury watches his daughter playing in the sandbox behind their suburban Chicago house, the thought that has flashed through his mind a million times in her two years of life comes again: Chloe will never be sick.

Not, at least, with the inherited form of colon cancer that has devastated his family, killing his mother, her father and her two brothers, and that he too may face because of a genetic mutation that makes him unusually susceptible.

By subjecting Chloe to a genetic test when she was an eight-cell embryo in a petri dish, Mr. Kingsbury and his wife, Colby, were able to determine that she did not harbor the defective gene. That was the reason they selected her, from among the other embryos they had conceived through elective in vitro fertilization, to implant in her mother_s uterus.

Prospective parents have been using the procedure, known as preimplantation genetic diagnosis, or P.G.D., for more than a decade to screen for genes certain to cause childhood diseases that are severe and largely untreatable.

Now a growing number of couples...