Gymnast Simone Biles’s signature move: Two backflips in the air, with straight legs, and a blind landing after an extra little half-twist.

Witnessing that gravity-defying feat, known as “the Biles,” you can’t help thinking that her level of talent is due to more than just thousands of hours of practice. Biles must have amazing genes. Clearly, there are some genetic markers for height–in Biles case, it helps to be small–but how about for factors like strength and precision?

For decades, scientists have been studying the role of nature and nurture in producing Olympic athletes like Biles. What they’ve learned is that while genes do play some role, they’re not a silver bullet. Environmental factors are extremely important: It makes a big difference if the athlete’s parents are interested in sports, and if they had access to both high-quality training and fresh food.

Scientists have pinpointed some gene variants that are associated with endurance and power, but are by no means predictive. “There is no super-athlete gene,” says Jennifer Kristin Wagner, a bioethicist at Geisinger Health System, based in Pennsylvania.

One gene that is frequently cited is the ACTN3 gene, dubbed the “speed gene,” which encodes instructions for making a specific kind of muscle protein. It’s very rare for Olympic athletes to lack the ACTN3 protein, but its importance shouldn’t be overstated, Wagner stresses.

“The speed gene comes up over and over,” she adds. “But it’s really limited as the proportion of muscle fibers is plastic and reactive to how you train.”

These limitations haven’t stopped companies from emerging that offer a way for parents and coaches to test their children for certain ACTN3 genotypes. Some test makers even provide reports that suggest a child might excel at some sports but not others.