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.