 Born in San Antonio, Texas, and raised in Waterloo, Iowa, Phelps was the eldest of four sons, all of whom became medical doctors or scientists, born to Gardner Dexter Phelps, M.D., a private-practice ophthalmologist, and Virginia Kuning Phelps.Phelps earned his undergraduate (B.A., 1959; Phi Beta Kappa) and medical (M.D., 1963; Alpha Omega Alpha) degrees at the University of Iowa in Iowa City.He completed an internship at Boston City Hospital between 1963 and 1964, and a residency there in internal medicine in the following year.