The Bethlem lines: genetic selection for high and low rearing activity in rats.

A foundation population derived from nine different rat strains was subjected to genetic selection for high and low rearing activity. A rapid response to selection was demonstrated, the two selected lines showing virtually no overlap in scores after 10 generations. Heritability estimates were assessed from parent-offspring regressions in the unselected control line and from comparisons of the achieved response with the selection pressure applied in the selected lines. Overall, the heritability of rearing activity in the rat was assessed to be about 0.25. In addition, a major gene effect was revealed, albinism being associated with low rearing activity scores.