Gene × environment correlation (rGE) occurs when genotype correlates (r) with probability of exposure to an environmental factor. Twin studies can address the existence of rGE by measuring the “genetics of the environment.” A gene-based example of rGE is the effect of CHRNA5 Asn398 to increase risk of lung cancer.38 This functional allele is associated with heavy smoking, thereby leading to increased exposure to carcinogens.
