When the former nurse Jamie Tyrone learned that she carried two copies of a gene called ApoE4, she “lost hope and direction,” and her “days were filled with fear, anxiety and sadness.” It meant that as she got older, she would likely develop Alzheimer’s disease, as her father had done before her.

The apoliprotein E gene, or ApoE, comes in three forms—E2, E3, and E4. The last one is the problem. People who carry one copy have a three-fold higher risk of Alzheimer’s than those with none. And those with two copies, as Tyrone carries, have 8- to 12-fold higher risks. Between 51 and 68 percent of them will develop the disease by the time they are 85. The risk is so large that some people who get their genomes analyzed (including James Watson, a co-discoverer of the DNA double helix) deliberately decide to redact their ApoE4 sequence. They’d rather not know.