Kassie Rose, 30 years old, faces a frightening prospect: if a genetic coin toss fails to go her way, she could lose her mind within a decade or two. A mutation that causes Alz­heimer’s disease runs in her family, the DeMoes of North Dakota. The odds of any DeMoe harboring the mutation are 50–50, and if the mutation is present, the chances of developing early-onset Alzheimer’s—the type that erodes memory before age 65—are 100 percent.