Common disorders are quantitative traits

After drifting apart for 100 years, the two worlds of genetics — quantitative genetics and molecular genetics — are finally coming together in genome-wide association (GWA) research, which shows that the heritability of complex traits and common disorders is due to multiple genes of small effect size. We highlight a polygenic framework, supported by recent GWA research, in which qualitative disorders can be interpreted simply as being the extremes of quantitative dimensions. Research that focuses on quantitative traits — including the low and high ends of normal distributions — could have far-reaching implications for the diagnosis, treatment and prevention of the problematic extremes of these traits.