Analysis of quantitative inheritance of body size in mice: V. Effects of small numbers of polygenes on similar genetic backgrounds

Sublines carrying isolated polygenes determining body size, developed by selection and repeated backcrossing to theLGandSMstrains of mice, were inbred by full-sib matings with concomitant selection to study the inheritance of body size, using birth, weaning (28-day) and 60-day body weights as indices.Mean body weights in those sublines backcrossed toSMand selected for large body size were greater than in those selected for small body size, proving that the difference resulting from seven earlier generations of backcrossing and selection, although small, was genetic and could be fixed by inbreeding. The mean body weights of the sublines selected for small body size drifted upward despite downward selection, a phenomenon thought to be due to the pressure of natural selection outweighing that of artificial selection.In the sublines developed from backcrosses to theLGstrain, mice from lines selected for small body size attained a mean body weight greater than that of mice from lines selected for large body size and also than of the parentalLGstrain. These results were contrary to expectation and a genetic interpretation was offered.