Reviews of the distribution of locus effect sizes show no difference between physiological and behavioural phenotypes (Flint et al. 2005). Moreover, in the most detailed analysis to date of the genetic architecture of complex traits in the mouse, among phenotypes there was no significant difference in the number or effect size of loci detected (Valdar et al. 2006). Intriguingly, regardless of the phenotype, the genetic effects that were detected explained about the same proportion of the additive variance, suggesting considerable similarity in the genetic architecture of many phenotypes (Valdar et al. 2006). Fig. 4 shows the effect sizes of 843 quantitative trait loci (QTL). The phenotypes include measures of anxiety and learning and memory, as well as haematology, immunology, biochemistry, physiology and anatomy. A full description is given in Valdar et al. (2006) and on a website (http://gscan.well.ox.ac.uk). Fig. 4 shows that preponderance of small effects. For each of the 100 phenotypes analysed, many loci contribute a small proportion to the variance. Large effect QTL are rare: only ten account for greater than 5% of phenotypic variance, and the mean is 2·2%.
