Selection for nursing ability and adult weight in mice.

Three selection treatments were conducted for 12 generations in each of two base populations (P and Q): (1) increased nursing ability of the mother (n12), as measured by mean 12-day weight of eight young within a crossfostering set (M(P) and M(Q) lines), (2) increased adult (42-day) body weight of the offspring (w42) (W(P) and W( Q) lines), and (3) performance combining the two traits (n12 and w42) into a selection index (B(P) and B(Q) lines). Lines C( P) and C(Q) were maintained as unselected controls in each population. In each line-generation subclass, 92 single-pair matings were made and the offspring assigned to balanced crossfostering sets of four dams each. Regression coefficients of mean performance (in grams) on generations were 0.080 +/-0.029 and 0.054 +/- 0.031 for n12 in M(P) and M(Q), and 0.680 +/- 0.039 and 0.868 +/- 0.051 for w42 in W(P) and W(Q), respectively. The B(P) and B(Q) lines showed genetic gains in n12 (0.090 and 0.053, respectively) and w42 (0.576 and 0.696) intermediate between the performance of M(P) and W(P), and M(Q) and W(Q), respectively, except for n12 of B(Q). Realized heritabilities for n12 were 0.16 +/- 0.05 and 0.11 +/- 0.06 and those for w42 were 0.40 +/- 0.02 and 0.43 +/- 0.03 for P and Q, respectively. The realized genetic correlations between n12 and w42 were 0.70 +/- 0.07 and 0.73 +/- 0.08 in P and Q, respectively. The ratios of the predicted to observed responses in M(P), B(P) and B(Q) were 0.99, 1.03 and 0.89, respectively. However, the predicted and observed responses differed in M( Q), W(P) and W(Q); the ratios were 1.29, 0.65 and 0.65, respectively. The observed combined responses for n12 and w42 in the index lines (B(P) and B(Q)) were smaller than the optimum expected from index selection. A possible cause was that the estimated genetic correlations (0.22 +/- 0.16 and -0.17 +/- 0.16 for B(P) and B( Q), respectively) and heritabilities (0.39 +/- 0.03 and 0.28 +/- 0.02, respectively) for w42 that were used to construct the selection index were smaller than the respective realized parameters.