Analyses of gene expression variation in yeast, rodents and humans concur in finding that the genetic architecture of gene expression is polygenic and that the genetic effects are relatively small (Morley et al. 2004; Brem & Kruglyak, 2005; Chesler et al. 2005; Hubner et al. 2005). In yeast, where we have the most reliable estimates, relatively few transcripts have large effects: the median effect size was equivalent to 27% of variation in transcript level, only 16% of loci accounted for more than 60% of variation, and a quarter explained less than 10% (Brem & Kruglyak, 2005). Although estimates of effect size are not as robust in rodents, the available data indicate that the effect sizes are comparable to those found in yeast (Morley et al. 2004; Chesler et al. 2005; Hubner et al. 2005).
