A fundamental objective in modern biology is the assignment of function to DNA sequence. This has been heightened by the completion of major genome sequencing projects and the advent of high throughput sequencing technologies. The accelerating output from genome sequencing projects provides a wealth of data for tackling fundamental biological questions and emphasises the importance of assigning function to DNA. A related and potentially more challenging objective is to understand the link between phenotype and genotype; this difficulty is in part due to the complexity of a trait. Phenotypes are determined at several levels, at multiple loci, the environment and interactions between the loci and the environment (Fig. 1a; [1]). Most phenotypes vary quantitatively across natural populations allowing insight to be gained from their study.