As mentioned previously, GWAS have been quite successful in identifying a portion of the genetic underpinnings of many common conditions and traits; thousands of variations, largely single nucleotide changes, have now been associated with a high degree of certainty with a wide range of conditions.12 GWAS have shed light on the pathogenesis of conditions such as asthma, diabetes, inflammatory bowel disease, and hypertension.24-28 However, it is important to recognize that markers found by GWAS generally confer only a very small increase in disease risk (with odds ratios in the vast majority of SNPs considerably less than 2), and are often not causally related to disease. Since the variations discovered by GWAS studies are often, even in aggregate, only weakly predictive of disease risk, GWAS have not yet had much direct impact on clinical care. This is in contrast to variations (mutations) known to be causal of rare disease. For most common conditions studied to date, family history, clinical characteristics and environmental factors account for a much larger proportion of measurable risk. As more is learned about genetic risk factors for
