What if sideline rage could be nipped in the bud with a quick genetic test that told Mom and Dad what sports – if any – Junior could master? The Boulder, Colo., company Atlas Sports Genetics today began selling just that sort of product: for $149, it says it will screen for variants of the gene ACTN3, which in elite-level athletes is associated with the presence of the muscle protein alpha-actinin-3. The protein helps muscles contract powerfully at high speeds, which may explain why the combination of ACTN3 variants that produce it has been found in Olympic sprinters.

The company's president, Kevin Reilly, tells ScientificAmerican.com that parents shouldn’t view the test as the final word on whether their child will excel at a particular sport. But, he says, it is more useful than physical tests in determining a child's athletic abilities before age 9.

At that age, "they don’t have the physical maturity and motor skills to do well," Reilly says. "That’s where the genetic test can come in [handy] for looking for early indicators of talent in performance areas.

"It’s a question of their motivation. This is a tool, not the tool," he says of consumers. "If they're relying on the genetic test as the only performance indicator to tell whether they will do good or bad in sports, they're going to be disappointed, because it's not for that purpose. If it’s a tool along with other components, you can use it to select what may be the best sport for you or for a child."

It takes about three weeks to get the results of the saliva test, which looks for three combinations of ACTN3 genes, with a child getting one variant from his mother and one from his father. (Reilly says that the Atlas Genetics screen is the only one commercially available in the  U.S. that tests for fitness-related genes.) Kids who have two copies of the X variant from both parents don’t make alpha-actinin-3, and might excel at endurance sports such as cross-country skiing, distance running or swimming, according to the company's Web site. Those with one copy of the X variant and one of the R variant will make some protein, Reilly says, and may excel at endurance or "power" sports such as soccer or cycling. And children with two copies of the R variant will make more alpha-actinin-3, setting them up for possible achievement in power or endurance sports including football, weight-lifting or sprinting.

We asked Stephen Roth, an assistant professor of exercise physiology, aging and genetics at the University of Maryland in College Park, to explain what is and isn’t known about the relationship between DNA and sports performance. Roth is a co-author of the Human Gene Map for Performance and Health-Related Fitness Phenotypes, a catalog of genes associated with sports-related fitness. The map was last published in 2006 in Medicine and Science in Sports and Exercise, a journal of the American College of Sports Medicine. The new edition will be available shortly.

This is an edited transcript.

To what extent do genes determine athletic ability?

Nobody knows the answer for sure and it depends on how specifically you define athletic ability. Most research suggests that genetics contribute significantly to sports performance, but it's very hard to put a number on. It's very hard to quantify football performance, for example. Most studies look at very specific endpoints: how much a gene contributes to muscle strength or maximal aerobic capacity, because those endpoints are very easy to measure from a research standpoint. If you try to parse it out, as much as 50 percent of muscle strength is determined by genetic factors.

The question is, what does that mean? To say there's some sort of heritable component to a trait tells us something can be passed on in a family that can contribute to performance, but what are the specific genes? How important, how predictive are those genes? We have no idea what is going on when it comes down to it. Some people are just genetically gifted, but we have just scratched the surface in defining what we mean by genetic advantage.

How many genes play a role in sports talent?

We don't know. I'm a co-author on a review published every few years where we catalog genes that have been studied in relation to performance. There are 200 genes we are cataloging as having some positive association with fitness-related performance … and there are 20,000 genes in the genome, so we're scratching the surface in relation to those studied.

Are those genetic factors just related to muscle strength, or do they show a variety of factors that are related to athleticism?

A wide range of factors. Because sports performance is so complex, we find muscle strength measures to metabolism performance measures or cardiovascular performance measures.

Atlas Sports Genetics is marketing tests for variants of the ACTN3 gene. Are there tests that pick up whether a person has other fitness-related genes?

ACTN3 is probably the most convincing of the genes studied so far, the most consistently associated [with sports-related fitness]. People who are the XX genotype do not have alfa-actinin-3 in their muscles. The idea is that in people who are lacking this protein, their muscles won't work as well and that will prevent them from reaching the upper echelon of power performance. That’s been indicated in a number of studies. But is the association about muscle fatigue? Contractile strength? As research starts to delve into these more refined traits, we don’t feel confident saying how the XX genotype is contributing to performance.

Another gene is ACE, which has been studied in relation to endurance performance. But the more these genes are studied, the messier the literature becomes. ACE is the most studied and is still a gene of interest, but we're trying to figure out if it's important and how — and the same question is reflected in ACTN3, but not reflected in ads for the test.

The ACE studies are more conflicting. It was originally argued that people with the II variant would be better at endurance and those with the DD variant would be better at strength. But the findings are not as consistent. When you break it down, we don’t see a clear story for how it would be working. If it does have a role, it’s a much smaller role than originally thought. There are larger question marks around ACE that would make it harder to sell as a test.

What can the results of the ACTN3 test tell us?

The results do tell you whether you have this protein in your muscle. That is clear. We have no idea if it contributes to performing at anything but an elite level. Even there, there are contradictions. We have very little information that it affects kids' performance. You may have a disadvantage in sprint performance, but it's likely you'll never see it except at an Olympic level. What 6- or 8-year-old cares about that?

Besides genetic testing, is DNA being used in other ways to promote athleticism?

The major issues out there are gene screening and whether we can predict performance or somehow tailor workout or training programs to particular people or select the sports they participate in in advance. The other is whether we can alter a genetic profile to enhance their performance. It's very similar to gene therapy in medicine. It hasn’t been successful in medicine and never studied in sports performance. It’s a real ethical dark zone, because there are medical concerns even pursuing it and no evidence that it would really work. Anti-doping societies have come out against it. It is definitely a concern. Technology is being developed in the medical arena. It won't take long for someone to push it in the sports world.

(This article was updated on Dec. 3 at 1:35 p.m. to correct that Medicine and Science in Sports and Exercise is a journal of the American College of Sports Medicine, not part of the Journal of the American College of Sports Medicine.)