Genetic Testing Challenges Medical Ethics By LINDSEY TANNER    CHICAGO (AP) -- Advances in genetic testing are placing strains on the principle of doctor-patient confidentiality, as doctors grapple with whether to tell a patient's relatives when the patient has an inherited disease or mutation that could afflict other members of the family.    In an article in Wednesday's Journal of the American Medical Association, doctors and legal experts argued against imposing a mandatory "duty to warn" nonpatients...