Suppose that the genes controlling a phenotypic trait have already been identified. How strong are the gene effects? Do some of the genes appear to have negligible impact on the trait, at least for the sample under study? In practice, it is rare that more than a few (say less than 5) genes would have noticeable effects in a modest sample (say 100 to 500 offspring). This section first considers analyzing the effect of a single gene, followed by multiple genes in an analysis of variance. Issues of model selection highlight the difficulty of detecting the significance of multiple genes even if their genotypes are known in advance.