But in addition to concerns about pigeonholing children before they can lace up their own cleats, scientists have lingering questions about the predictive value of certain genetic tests. As Carl Foster, director of the Human Performance Laboratory at the University of Wisconsin–La Crosse and coauthor of several ACTN3 studies, points out, there is more to an all-star than just a few specific genes. In 2007, Foster coauthored a study of an Olympic long jumper from Spain who was found to have two X variants of ACTN3. “Yet he obviously found a way to be pretty springy,” Foster says. Foster estimates that there are at least 30 genes that relate to high-level athletic performance. Even if we knew them all, the relationship between genetics and training would remain a tangled skein. “If you want to learn if your kid is a good sprinter,” Foster says, “the most efficient way is to take him to the playground and have him race the other kids.”

Nonetheless, the march of personal genetic testing will continue, bolstered by the new nondiscrimination law as well as by technology that is rendering tests ever cheaper and quicker. According to Brian Naughton, one of 23andMe’s scientists, the cost of a full genome scan is about to drop from approximately $100,000 into the $10,000-to-$20,000 range and will sink even further over the next few years. “Certainly in the next five years we’ll see more full sequencing sold directly to consumers,” Naughton says. “Once you have a full sequence, that’s really as far as your genetics goes.” How far genetics goes toward helping a customer know thyself, however, is a question that will endure.

David Epstein is a staff writer for Sports Illustrated. He frequently writes about sports science and medicine for the magazine, and in February, he coauthored the story that revealed Yankees third baseman Alex Rodriguez had used steroids.