The idea that genes alone will make a major contribution to understanding racial disparities in health is not consistent with the available evidence.137 First, there are differences in genetic susceptibility in human populations but they are unlikely to be strongly patterned by race.123 The genetic alleles that predict susceptibility to common diseases tend to be present in many populations, although at varying rates. Second, many genes are likely to be involved in most common chronic illnesses and each gene is likely to have many variants.138 Thus, the chronic diseases that make a large contribution to the burden of disease in contemporary society are seldom driven by any single risk factor, whether genetic or environmental. Third, the overall contribution of genetics to population health is likely to be modest. A review of the more than 300 replicated associations that have been identified for more than 70 diseases from genome-wide association studies concluded that their effect on disease risk is very modest, with an identified variant typically increasing disease risk by 10 to 30%.139 Thus, genetic effects are small and hard to detect and there are few examples of gene-environment interactions for modest gene effects or small environmental effects.
