Map Manager QTXb20 free software [13] was used to perform linkage analysis and the permutation test. Ninety percent of the mouse genome was within a 20 cM intermarker distance. The marker map was generated using the Kosambis map function and 1,000 permutations were performed for every phenotype (P < 0.05). The permutation tests were carried out to establish empirical significance thresholds for the interactions. A threshold equal to or above the 37th percentile (P = 0.63) was considered suggestively significant, and the level for the significant threshold was set to the 95th percentile (P = 0.05). Interval mapping was made with a 2 cM increase under the additive regression model in order to calculate the test statistics.