Genetics of Body Composition in Mice'

Selection in mice for high and low fat percentage and high and low lean percentage, based on indicator traits of total composition, point to moderately high heritabilities which are in agreement with base population estimates calculated as regressions of sons on sires. Selection for a single fat depot, epididymal fat pad weight as a percentage of body weight at 12-weeks of age, resulted in correlated changes in all fat depot percentages. However, the selected fat depot showed a higher rate of response than other fat depots, leading to a redistribution of fat tissues and suggesting that a local control of adipose tissue deposition occurs. Undesirable correlated responses to selection for reduced fat percentage were a lower growth rate and no change in feed efficiency. Several selection index criteria were used in an attempt to overcome some of the difficulties with undesirable correlated responses encountered with single-trait selection. The selection index criteria had moderately high heritabilities, but in many cases responses of the component traits did a_otagree with expectation. It is, therefore, essential to monitor-closely the correlated responses in component traits of a selection index, particularly when antagonisms exist between the sign of the genetic correlation and the direction of selection for pairs of traits.