GENETIC-BASIS OF RESPONSE TO 50 GENERATIONS OF SELECTION ON BODY-WEIGHT IN INBRED MICE

A long-established inbred strain of mice was divergently selected for body weight for 50 generations. Selection of new mutations affecting t he trait eventually led to a divergence of approximately three phenoty pic standard deviations between the high and low lines. Heritability f or body weight increased at a rate between 0.23% and 0.57% per generat ion from new mutations, depending on the genetic model assumed. About two-thirds of the selection response was in the upward direction. The response was episodic, suggesting a substantial contribution from the selection of mutations with large effects on the trait. A maximum like lihood procedure was used to estimate the number of factors contributi ng to the response using data from line crosses, with models of n equi valent gene effects (i.e., to estimate the Wright-Castle index), or n genes with variable effects. The results of the analysis of data from a cross between the selected high line and an unselected control line indicated that two major factors were involved, with the suggestion of an additional minor factor.