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...