With early Alzheimer's in the family, these sisters decided to test for the gene

Karen Douthitt (left) and her two of her older sisters, Susie Gilliam (center), and June Ward (right) each took a test for the genetic mutation presenilin 1 after their mom got Alzheimer's disease in her early 60s. Each child of a parent with this mutation has a 50% chance of inheriting it. (Juan Diego Reyes for NPR)

In some families, Alzheimer's disease seems inevitable.

"Your grandmother has it, your mom has it, your uncle has it, your aunts have it, your cousin has it. I always assumed that I would have it," says Karen Douthitt, 57.