Despite a high heritability estimate from the inbred strain experiment, the response to selection for head dipping proceeded at a slow rate and resulted in a modest difference between HEB and LEB mice of both replicates, with a realized heritability estimate of 0.13 for each replicate after five generations of selection. In retrospect, the modest response is most likely due to the choice of the C57BL/6J and DBA/2J progenitor strains. Although moderate differences were observed between these two strains in our first inbred strain comparison, this difference was not found in the second, which indicated in the aggregate that only modest additive genetic variance relating to head dipping existed in the starting population. Given the slow response to selection, it is probable that many genes, each exerting a small influence, are contributing to the trait. It does not appear that there are a few genes with large effect on head dipping present in the cross, or else a much more rapid divergence would have been expected.
