WHEN, in 1996, French nun Mariannick Caniou found out she didn’t have Huntington’s disease, the lethal, degenerative genetic disorder, she fell into a depression. Throughout her life, she had been convinced that she would develop the illness that had killed her mother and grandmother. So convinced, in fact, that all her most important decisions had been based on that conviction: her decision not to marry, for example, or not to have children. She didn’t regret her decision to enter the religious life, but now she had to wonder if the specter of Huntington’s had haunted that too: “Everything I had built, my life, seemed no more substantial than air…”

This article first appeared in Slate on 29 September 2017. To continue reading, click here.