t is fall in Pennsylvania's Lancaster County, and Jacob Stoltzfoos is working his field much the way his forebears did three centuries ago--tugging at the yoke of a Belgian draft mule. The only sounds he hears are the snap of a rein across the mule's hindquarters, the simple mechanical whirl of his corn-harvesting machine and the creak of his oak-plank wagon as he hauls another stack of feed corn to his son-in-law's silo. Like their ancestors, Jacob and his kin light their farmhouses with gas lanterns and drive carriage horses--never automobiles--back and forth to town.

Jacob and his family are Old Order Amish. They belong to a community that lives at the edge of American society, with spiritual values rooted in Pennsylvania-Deutsch soil. Most of the 20,000 Old Order Amish living in Lancaster County are descended from the 200 German-Swiss farmers who immigrated there during the 18th century. The community has paid a price for its separateness: because the Amish forbid marriage outside the Old Order, centuries of inbreeding have afflicted them with certain genetic diseases that strike their young in alarming numbers.

"Twelve disorders that I see here are founder-gene defects carried by the dozen families that established this population 300 years ago," observes Dr. D. Holmes Morton, 47, a pediatrician and geneticist who gave up an academic career to work among the Amish. One of those diseases, he has discovered, is glutaric aciduria, a metabolic deficiency that usually strikes children between the ages of 6 months and 5 years. Often triggered by childhood illnesses such as chickenpox or strep throat, it can cause permanent brain injury that can lead to chronic disability, medical complications and even early death.

Morton's introduction to glutaric aciduria and the Amish came one night in 1987 while he was on duty in the clinical laboratory at Children's Hospital of Philadelphia. A fellow physician, Dr. Charles Nichter, asked him to analyze the urine sample of an Amish child, Danny Lapp, from Lancaster County. At the time, Danny was alert but had no control over his arms or legs--signs of cerebral palsy, which was Nichter's medical specialty. Morton's testing revealed a metabolic fingerprint that could be caused only by glutaric aciduria, a disorder that had previously been reported only eight times worldwide--and not once in Lancaster County.