So what, exactly, do a few milliliters of spit tell you? The most important thing you'll learn is what kind of genetic markers you carry.
A genetic marker is any alteration in your DNA that may indicate an increased risk of developing a specific disease or disorder. Because SNPs are, by their very definition, variations in DNA, they can be used as flags or markers for nearby DNA that affects your health.
Different service providers identify different genetic markers. For example, 23andMe identifies your risk for developing lupus, while deCODEme and Navigenics don't. These latter companies, however, test for markers of Alzheimer's disease, which isn't included in 23andMe's repertoire. The table below compares the service providers across 10 other important diseases and disorders [source: Genetics and Public Policy Center].
Understanding your susceptibility to certain diseases is a powerful tool. Say, for instance, your at-home genetic test reveals you're predisposed to develop osteoporosis. You can use this information to take a more proactive role in your own health care. You might decide to take supplements to ensure you're getting enough calcium and vitamin D. You might also engage in regular weight-bearing exercise and opt to have a bone density test to determine your risk for future fracture. These kinds of decisions are part of a growing trend that finds more patients -- well educated and computer-literate -- assuming control of their own health care.
Better health is not the only thing you can get out of your spit. You can also trace your ancestral roots. This is possible because closely related individuals have more similarities in their DNA. By comparing your genetic information to that of people from around the world, you can fill out a comprehensive family tree, tracing your lineage through either your mother or your father.