For example, almost every champion runner has what is known as the R allele of the ACTN3 gene. It produces a protein that builds fast-twitch muscle fibers, and it is also associated with improving strength and recovery from muscle injury.8 Someday it may be possible to edit this variation of the ACTN3 gene into the DNA of your kids. Would that be unfair? Is it unfair that some kids are born with it naturally? Why is one more unfair than the other?
