The head dipping scores for the B6D2F3 founding population were normally distributed with a range of 21–116 head dips (data not shown), a range similar to that seen for the inbred strain means in this task. Thus, there was a good deal of variation in the starting population from which to select for high and low head dipping. Small differences between high exploratory behavior (HEB) and low exploratory behavior (LEB) mice were apparent after one generation of selection (see Fig. 2A and B; generation S1, combined R2 = 0.03) and were larger after two generations (R2 = 0.11). By the fifth selected generation, selected line accounted for 15% of the total variation. The selection parameters at each generation of selection are shown in Table 7. The realized heritability estimate was slightly larger in both upward replicates (h2 = 0.14 for HEB-1 and 0.20 for HEB-2) than in the downward direction (h2 = 0.11 for LEB-1 and 0.04 for LEB-2). Heritability for the divergence between HEB and LEB mice was estimated as 0.13 for both replicates.
