For $399 and a glob of saliva, 23andMe Inc. -- the name is a reference to our 23 chromosomes -- will probe small sections of your genome that have scientifically documented connections not only to diseases but also to personal traits. Ever wonder if you don’t like broccoli because your genes make you sensitive to bitter foods? (Just think of how handy that excuse could have been when you were a kid.) Or how well your nose hones in on the smell of sweat? (Might explain that blind date.) Curious about your pain tolerance? (And your dentist thought you were just a wimp!) And who hasn’t wondered about the texture of their earwax? (If you’re Caucasian, it’s probably wet.) Some of the results -- like those of the test for a gene variant that determines how well one metabolizes caffeine -- can actually be useful. When Peter R. Orszag, the White House budget director, began to worry about how much coffee he was drinking as he puzzled over the nation’s economic crisis, he took a genetic test, and it informed him his body could deal with large doses. So Orszag kept right on slurping. The rapidly growing list of traits that scientists have tied to particular genes has given rise to an industry that bypasses the doctor’s office and markets genetic tests directly to consumers. Whether that industry’s marketing is ahead of its science, though, is a point of hot debate.

The ACTN3 gene provides an aptly controversial example. A particular version of the ACTN3 gene -- and we all have a pair of ACTN3 genes -- known as the R variant directs the body to produce a protein found in muscles used for explosive movements, whereas the X variant prevents the protein from being produced. In 2003, a study of elite athletes found that those in speed and power sports are far more likely to have R variants than other people are, and that athletes in endurance sports are more likely to have X variants. For $149 and a simple cheek swab, Atlas Sports Genetics in Boulder, Colorado, will test a child of any age -- they say it’s most useful to test children younger than eight -- for ACTN3 variants so that a parent can accordingly nudge the youth toward an appropriate sport. Atlas’s sales pitch proclaims that “knowing what a person is born with can ensure they develop into the best athlete they can be.”