The Manipulation of Variation in Quantitative Traits: A Review of Possible Genetic Strategies

Sample results from the literature are presented to illustrate the likely efficacy of genetic strategies to influence intrapopulation phenotypic variance. Inbreeding can increase uniformity for simply inherited and highly heritable traits, but inbreeding often leads to increased variability for economically important quantitative traits. There is little evidence that topcrossing, the mating of inbred males to random bred females, reduces phenotypic variance in the offspring generation. Outbreeding can increase phenotypic uniformity of F1 individuals. Advanced generations of inter se mating, following an initial cross, can cause increased phenotypic variance. In neither previous case, though, is the effect universal. Data are limited as to whether positive and negative assortative mating lead to expected increases and decreases, respectively, in phenotypic variance. There is evidence that variability of an individual's offspring is heritable, but detecting and utilizing such genetic variation to select for uniformity (or variability) would be slow and expensive. Long-term directional selection, in laboratory animal populations, usually leads to increased phenotypic variance for the selected trait. Also, in laboratory animal experiments, long-term stabilizing selection can decrease genetic and phenotypic variance, but it would not likely be a practical approach in agriculturally important animals. If large numbers of individuals could be cloned from a single genotype, phenotypic variation for economically important quantitative traits could be much reduced.