Measuring the effects of genes and environment on complex traits.

Complex diseases and traits are influenced by a combination of genetic and environmental risk factors, some of which may be known, and many of which are unknown. It is possible to estimate the relative importance of the influence of genes and environment on a trait by studying correlations in the trait in related individuals. Known risk factors can be measured and included in the statistical models to understand disease etiology better. The joint effect of specific genes and environmental exposures can be estimated by measuring these in individuals, not necessarily related, with and without the disease of interest or with a range of trait values. These methods are illustrated by considering two example analyses in detail. The first is an analysis of a study of adolescent twins, quantifying the effect of genes and environment, including measured sun exposure, on the density of nevi. The second is an analysis of a case-control study, examining the joint effect of the GSTT1 gene and vegetable intake on risk of colorectal cancer.