Genetic Future discusses the ACTN3 gene. There appears to be a link between this gene and sprinting success, but as Daniel MacArthur points out in the piece, there much more to it than just that one gene.

ACTN3 is just one of many factors influencing athletic performance At the highest levels of performance ACTN3 genotype certainly make a big difference: among Olympic-level sprinters the frequency of individuals carrying two disrupted ACTN3 copies is vanishingly low (less than 3%, compared to ~18% in the general population). However, this large effect is due to the exceptionally strong selection that occurs during the slow climb to the Olympic level. The vast majority of athletes who start that climb will never make it to the top; those who do will be the tiny minority who have nearly everything in their favour, including the right genes.

So super-elite athletes need to have the right ACTN3 combination, but they also have to have a whole host of other factors working in their favour - this one gene is just a minor ingredient in a large and complex recipe. In fact, most studies performed so far suggest that ACTN3 explains just 2-3% of the variation in muscle function in the general population. The rest of the variation is determined by a wide range of genetic and environmental factors, most of which (particularly the genetic factors) are very poorly understood.

Someday, however, we're likely to get to the point where these genes are well understood. At that point, we'll likely be discussing gene doping (for lack of a better term). For example, would it be wrong to inject the protein(s) created by the ACTN3 gene if your copies are both disrupted? Would that be unethical, or is the person taking the protein just leveling the playing field?