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An experimental evaluation of the effect of mating systems and selection upon an additive trait thought to be highly heritable was made. There were two similar replications. Each consisted of a mass selected and randomly selected group, with five mating systems within each group.Realized heritabilities in the mass selected lines were considerably less than was expected prior to the initiation of the experiment, and averaged approximately fourteen percentage points less than heritability estimated from the zero generation. This in turn resulted in smaller correlations between the genotypes of mates than had been previously expected in the assortatively and disassortatively mated lines.The average response of the mass selected, assortatively mated lines was slightly more than the mass selected, randomly mated lines, though not statistically significant. This result seems to conform to theoretical expectations.In the mass selected lines, estimates of phenotypic and genetic variance declined regardless of mating systems. There was a tendency for phenotypic variances to decrease in the randomly selected lines, but this was not the case for estimates of genetic variance.As an aid to selection, it seems that assortative mating would be of little value with traits of low or intermediate heritability but might be useful if the trait is highly heritable.

Body weights and tail lengths were observed every 3 days from birth to 60 days of age and every 6 days from 60 to 96 days in four lines of mice: C57BL/6J, an inbred; J, a synthetic outbred; GR, Goodale large body size line; and FR, Falconer large body size line. Mean 96-day body weights for lines C57BL, J, GR and FR were 25.4, 29.7, 48.8 and 49.1 gm for males, and 19.9, 23.8, 38.5 and 38.2 gm for females, respectively. Lines GR and FR gave identical body weights at all ages studied. Both of these lines had previously plateaued in response to selection for large body size. The variability in body weight was smallest for lines C57BL and J, intermediate for FR and highest for GR. The pattern of variance over time was very similar in all lines and both sexes, showing a minimum at birth and a maximum at age of inflection. Growth in tail length of the four lines showed similar between line differences except that length in GR was greater than in FR. Age at vaginal opening in females coincided closely with age of inflection in body weight growth. Age at point of inflection did not differ between lines but appeared to occur somewhat earlier in females than in males.

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