Figure 1. MSTN genotype distributions among Thoroughbred horses. Distribution of a. C/C, b. C/T and c. T/T genotypes among (n = 79) Group race winning Thoroughbreds. The non-uniformity of the distributions (b. and c.) may be explained by the absence of 9 f races among the 84 Group 1 races held in Great Britain. d. To avoid the bias introduced by racing distances for two-year-olds limited to ≤8 f (in Great Britain and Ireland), the distribution of genotypes was plotted for individuals with best race distance >8 f and for those two-year-olds that won their best race ≤8 f mean three-year-old race distances were used. e. Genotype distributions among National Hunt racing Thoroughbreds (aqua) f. Genotype distributions among Flat racing Thoroughbred (orange) and Egyptian (purple) horse populations. a.-e. C/C - blue, C/T - red, T/T - green, vertical axes indicate frequency, horizontal axes indicate best race distance (furlongs). The International Federation of Horseracing Authorities recognizes five race distance categories: Sprint (5-6.5 f, ≤1,300 m), Mile (6.51-9.49 f, 1,301-1,900 m), Intermediate (9.5-10.5 f, 1,901-2,112 m), Long (10.51-13.5 f, 2,114-2,716 m) and Extended (>13.51 f, >2,717 m); S-M-I-L-E [Note: 1 furlong = 1/8 mile = 201.2 meters].