BRITTON, S.L., and L.G. KOCH. Animal genetic models for complex traits of physical capacity. Exerc. Sport Sci. Rev., Vol. 29, No. 1, pp 7–14, 2001. Understanding the genetic basis for variance in complex physical traits such as aerobic capacity has become an attainable goal. A starting point is the development or identification of animal genetic models that contrast the low and high values for the trait of interest. Genes that cause natural trait variation can ultimately be determined from animal models via genetic linkage.