Long-term selection for antagonistic traits

Within family selection for 3-5 week gain (G3-5) and against 8-week weight (BW8) was practiced for 54 generations in a mouse line of eight full-sib families per generation. The genetic correlation was 0.56 between the two traits, and rate of selection was intended to be 25 %. The selection index derived from the realized parameters of the first generations of the experiment was -0.63 × BW8 + 1.0 × G3-5. Selection was successful insofar as it changed the traits in the desired directions, but this was achieved at considerable costs, i.e. gains were 10 to 15 % of gains of direct, unimpeded selection.