Genetics and fast-twitch muscles the gene actn3 encodes a protein that functions in fast-twitch muscles. people can be classified according to which genotype of this gene they have, rr, rx, or xx, and also according to which genetic allele they have, r or x. the study1 described here, examines the association between this gene and different subgroups of the population. all participants in the study live in australia. the earlier exercises only included the control group. testing genotypes for fast-twitch muscles the study on genetics and fast-twitch muscles includes a sample of elite sprinters, a sample of elite endurance athletes, and a control group of non-athletes. is there an association between genotype classification (rr, rx, or xx) and group (sprinter, endurance, control group)