Background

Complex genetic diseases are defi ned as those infl uenced by multiple genes and by environmental eff ects. In the past, individual genetic variants contributing to the risk of disease were usually not known, so the contribution of genes to disease was recognised through increased risk of disease in relatives of aff ected probands. Modeling allowed the genetic component of disease to be expressed as variance components and heritabilities. However, with the advent of genome-wide association studies (GWAS), individual genetic risk factors, or at least markers linked to them, are identifi able. Th is provides a description of the genetics in quite diff erent terms to the traditional use of variance components. Th e new description is based on the frequency of individual risk alleles and their eff ect sizes expressed either as the relative risk or the odd