The genetic basis for adaptations to the diverse environments encountered by a wide-ranging species should be reflected in the phenotypes of hybrids between differentiated populations. We crossed mice from two ecologically different populations to determine whether adaptations to seasonality in Peromyscus leucopus, the white-footed mouse, display directional dominance or whether they are affected by specific interactions between genes. Connecticut mice (C) have many adaptations to seasonality that are reduced or absent in Georgia mice (G) and that affect both reproductive and thermoregulatory traits. Because these adaptations are cued by photoperiod, temperature, or both, parental and hybrid mice were acclimated to 13 degrees C in either long-day or short-day photoperiod, and several morphological, reproductive, and thermoregulatory traits were measured. Several traits, especially those involved in response to the environment, such as nest size at 13 degrees C, reproductive regression under short-day photoperiod, and molt to winter pelage, showed a non-Mendelian pattern of inheritance. However, there was a pronounced difference between maternal lineages in the F2 generation in that C-maternal F2 mice were less responsive to short-day photoperiod and cold than were the G-maternal F2 mice. Because these two classes of F2 mice are genetically equivalent, this breakdown cannot be explained as a disruption of epistatic gene interactions unless recombination rates are higher in the C-maternal lineage.