Figure 1: Genetic epistasis.

Interactions among a set of genes can be sensitively measured by constructing trans-heterozygotes between mutant alleles a and b (that is, by generating an individual of genotype a +/+ b) of different loci and then scoring them for quantitative phenotypes, such as behavioural scores. (Quantitative traits are influenced by a large number of genes, often of small individual effect.) An interaction is deemed to be significant if the observed score deviates statistically from the expected score. Expected scores are calculated on the basis of a measure of the average interactivity of each allele ('general combining activity'²⁹) with all of the others.