In just a year’s time, GWA studies have come to dominate the gene-hunting literature. The ‘A’ of GWA is association, which is simply a correlation between a particular allele and a trait in the population. The trait can be a qualitative trait like an ADHD diagnosis or a quantitative trait like hyperactivity symptom scores. For example, one of the best documented associations involves a particular allele of the apolipoprotein E gene on chromosome 19 which is associated with late-onset Alzheimer’s disease. In dozens of association studies, the frequency of allele 4 was found to be about 40 percent in individuals with Alzheimer’s disease and about 15 percent in controls. Although association is a correlation between DNA sequence variation and a trait, this correlation implies causation because DNA sequence is not changed by behaviour or the environment.
