Selection in guinea pigs: III. Correlated responses to selection for litter size and body weight.

Correlated responses in number born, number born alive, number weaned, birth weight, weaning weight, 13-wk weight and index were evaluated in a guinea pig population after four generations of single trait and index selection for number born alive and 13-wk weight. Regressions of correlated response on generation number in the 13-wk weight line were all positive, but were significant only for birth weight (5.65 +/- .66 g) and index (25.58 +/- 9.90). In the litter size line, significant responses were found for 13-wk weight (21.17 +/- 5.34 g) and birth weight (.80 +/- .21 g). All other responses in this line, except number weaned (-.05 +/- .08), were positive but nonsignificant. There were no significant trends in the index line. Regression coefficients from weighted multiple regression analyses suggested significant improvement in number born (.07 +/- .002), number born alive (.006 +/- .002) and number weaned (.01 +/- .001) in the 13-wk weight line. Regression values in the litter size line were .03 +/- .001 and -.04 +/- .001 for number born and number weaned, respectively. The other regressions were positive but nonsignificant except for weaning weight, which was nonsignificantly negative. None of the correlated responses to index selection was significant. Realized genetic correlations between 13-wk weight and number born alive and index were .37 +/- 1.54 and .31 +/- .98, respectively. The correlation between index and number born alive was .19 +/- 5.18.