ACCUMULATION OF MUTATIONS AFFECTING BODY-WEIGHT IN INBRED MOUSE LINES

The variation from spontaneous mutations for B-week body weight in the mouse was estimated by selection from a cross of two inbred sublines, C57BL/6 and C57BL/10, separated about 50 years previously from the sa me inbred line. Selection was practised high and low for 12 generation s from the F-2, followed by one generation of relaxation. The lines di verged by approximately 1.7 g or 0.7 sd. The additive genetic variance was estimated in the F, by restricted maximum likelihood and from the selection response, and from this variance the mutational heritabilit y h(M)(2) was estimated using the number of generations since divergen ce. Estimates of h(M)(2) range from 0.08 to 0.10% depending on the met hod of analysis. These estimates are similar to those found for other species, but lower than other estimates for the mouse. It is concluded that substantial natural and, perhaps, artificial selection operated during the maintenance of the sublines.