Crossing selected lines vs. selecting from a cross in mice

Eight generations of selection were practiced for post-weaning gain in body weight and for litter size at birth in two outbred stocks of mice and in a stock derived by crossing these outbreds. At each generation, the selected outbred stocks were crossed, and offspring of the cross compared to those from selection in the cross stock. Selection from the cross stock gave greater response than crossing selected stocks in post-weaning gain, a trait where inheritance was primarily by additive gene action. For litter size, a trait where dominance and epistatic interactions were of greater importance, no clear advantage resulted from crossing prior to selection.