What this means

ACTN3 codes for alpha-actinin-3, a structural protein specific to fast-twitch (type II) muscle fibres. The R577X variant introduces a premature stop codon: people with two X copies make no functional alpha-actinin-3, and a related protein (alpha-actinin-2) takes over. About 18% of the world's population are XX and walk around perfectly athletically. The variant is slightly enriched among elite sprinters (RR) and slightly depleted among them in endurance disciplines (XX), but the effect is small and training swamps it in any individual case.

The ACTN3 gene makes a protein found only in your fast-twitch muscle fibres — the ones you use for explosive movements like sprinting or jumping. A common DNA change switches this gene off, and about one in five people worldwide make none of the protein at all. They're perfectly athletic; a related protein steps in to do the job. The "on" version shows up slightly more often in elite sprinters, the "off" version slightly more often in endurance athletes. The effect is small enough that training, technique, and grit completely overshadow it.

Caveats

• The effect is statistical and small — training, technique, and grit dominate.
• About 1 in 5 people are XX and they include plenty of elite athletes.
• This SNP is most studied in European-ancestry cohorts; XX frequency varies by population.
• This is not a guide to choosing a sport.

References

• Yang et al. — ACTN3 genotype is associated with human elite athletic performance (AJHG, 2003)
• MacArthur & North — A gene for speed? The evolution and function of alpha-actinin-3 (BioEssays, 2004)