Alexis Tappan has a college degree, a full-time job and a slew of church, community and other everyday involvements. All of that shows how she’s defied an ominous, years-ago prognosis. At birth, she was critically ill. Doctors said she’d never walk, talk or develop normally. They warned her parents she could die young from sickle cell disease. Historically, patients often died in childhood from the disease’s damage to the body’s organs, bones and brain, including strokes.