You carry two copies of all of your genes — one from each of your parents. 23andMe's latest test is designed to tell you if your chances of developing a disease are higher than those of the average person. To do that, it scans your DNA for genetic mutations, or tweaks in your genetic material, that have been linked with specific diseases. If it finds one, it adds that information to your profile.
Before you freak out, you should know that having a mutation does not necessarily mean you will develop that illness. It simply means you're more likely to get it than someone without that genetic tweak. In other words, "you could have the mutation and not get it, or you could not have the mutation and get it," says Klitzman.