The efficient identification of the genes that influence quantitative traits requires: large sample sizes; the analysis of large numbers of polymorphisms in and around genes or surrogates for these; repeated testing in independent samples; the realisation that the inheritance patterns of quantitative trait loci will show the full range of effects found for genes that affect discrete traits; and choosing the appropriate genetic structure of the sample and the kind of DNA polymorphism for the different stages in the identification of the quantitative trait loci. The choice of trait is critical to the successful production of diagnostic tests. Since this is the most important single factor affecting whether a test will be commercialised, not only due to the economic importance of the trait, but whether there are easy, alternative methods to improve the trait that are cheaper to implement than a DNA test.