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Selection for a Ratio of Body Weight Gains in Mice¹

U. S. Department of Agriculture, West Lafayette, Indiana 47907 and Purdue University, West Lafayette, Indiana 47907

Abstract

A replicated eight-generation selection experiment for increased early post-weaning gain (G_21–42) (line 1) and increased ratio (R) of early post-weaning gain to total post-weaning gain (G_21–63) (line 2) was conducted with mice. The average number of mice per line-generation-replication subclass was 300.

There was a consistent response to selection in the form of increased gain from 21 to 42 days of age in line 1. Realized h² was 0.25 and 0.20 in replicates I and II. Also, there were increases in gain from 42 to 63 days of age. In other words, increased early post-weaning gain also resulted in increased late post-weaning gain and increased adult body weight. Increasing gain from 21 to 42 days of age did not increase R.

In line 2, there was little or no change in gain from 21 to 42 days, but gain from 42 to 63 days and 63-day body weight did decrease. Therefore, the percentage of post-weaning gain occurring in the 21- to 42-day period increased 4.3 and 12.3 percentage points in replicates I and II, respectively. The realized h² of R was 0.05 and 0.13 in the two replicates. These data indicate that selection did increase the percentage of 21- to 63-day gain in mice that occurred in the 21- to 42-day period and did change the growth curve. However, the rate of change was quite slow even when all selection pressure was directed to this one trait. The average realized genetic correlation between the two selected traits was 0.01 indicating independence of genetic material controlling the two traits. Those of R with BW₆₃, and G_21–42 with BW₆₃ were –.31 and 0.60.

¹ Journal Paper No. 5014, Purdue Agricultural Experiment Station.

² Northcentral Region, A.R.S.; Department of Animal Sciences, G-117 Life Sciences Building, Purdue University.