We all have a gene called ACTN3, but certain variants of it help our bodies make a special protein called alpha-actinin-3. This protein controls fast-twitch muscle fibers, the cells responsible for the speedy tensing and flexing of the muscles involved in sprinting or weight-lifting.
This discovery, which happened around 2008 when geneticists studying elite sprinters and power athletes found that very few among them had two defective ACTN3 copies, is what led to the gene being dubbed the "sports gene."
Among the general population, however, some 18% of us are completely deficient in the speedy-muscle-contracting protein — we inherited two defective copies of ACTN3.