(Left) Two parental strains of mice and their F1 hybrid. (Right) Data from McCall and Frierson (1997) Behavior Genetics 27(3):181-190 showing running time to fatigue under hypoxia (1/2 atm) in the three strains. At 8 weeks of age, mice were exposed for an additional 8 weeks to either normoxia (N) or hypoxia (H). These are the 6 treatment groups used in most of our studies. The F1 hybrids have an extraordinary capacity for hypoxic treadmill exercise following acclimation to hypoxia, and some individuals ran for >2h (40 cm/s, 15° incline, ½ atm P). The differences in performance can be principally attributed to only two-loci of major effect, as well as an unknown number of modifying genes. Data are means +/- sem.