Heredity of muscle fibre composition estimated from a selection experiment in rats.

The extent to which muscle fibre composition was determined by the genes transmitted from parents was estimated by using successive selections of rats. A foundation population (G0) was established to carry out the selections; it consisted of 100 albino rats which were randomly chosen from heterogeneous stock. The heterogeneous stock was produced by random-mating of three strains, Wistar-Imamichi, Fischer 344 and Donryu. Successive selection for a high percentage of slow twitch fibres (% ST) was made from G0 to the fourth generation (G4). The mean values of % ST changed from 50.0% in G0 to 55.6% in G4. The mean value in G4 was significantly higher than that in G0. The realized heritability in G0-G4 was calculated from the regression of selection response on the cumulated selection differential. The regression coefficient was 0.17 (SEM 0.04). The realized heritability was significantly different from 0 at the 0.05 level. We concluded that in rats about 17% of the variation of muscle fibre composition was determined by the genes transmitted from parents.