The limits to artificial selection for body weight in the mouse: IV. Sources of new genetic variance—irradiation and outcrossing

Two methods are examined of introducing new genetic variance into a line of mice selected for high 6-week weight which, at its limit, displayed no additive genetic variance. 2. The first method—irradiation—gave largely negative results. Any further gain under selection that was achieved could not be clearly distinguished from a possible environmental trend. 3. The second method—outcrossing to an unselected strain and then selecting from the cross—resulted in a clear gain over the original limit, but nine generations were required even to recover the original limit. 4. Various methods of transcending selection limits are evaluated in terms of their application to livestock improvement.