The email was the opposite of scary. Subject: “Your Genetic Profile is Ready at 23andMe!” Six weeks earlier, I had mailed the genetic testing company a tube of my own spit. Now it was time for me to “start exploring” the results. I was terrified.

I read a lot about genetics, so I knew that this sort of test isn’t usually very useful, health-wise. 23andMe estimates your risk of getting common diseases — heart disease, diabetes, asthma, and many others — based on large studies of people who carry the same DNA-letter blips as you do. Think of a chromosome as a long string of DNA letters. According to one study, people who have a ‘T’ on one