of other studies have found the effect of the G allele to be recessive, with G/G individuals requiring more morphine or fentanyl to achieve analgesia [30-34]. The potentially contradictory findings of both dominant and recessive effects for A118G may be the result of patient ethnicity. A118G has a minor allele frequency of 38% in Asians, 16% in Europeans, and 3% in African-Americans (http://www.1000genomes.org). The lower frequency in Europeans and African-Americans results in only a small percentage of individuals carrying the G/G genotype, potentially making studies of these populations underpowered for an analysis of a recessive model. This issue would not be likely to arise in Asian populations, in which a recessive effect of the G allele has been repeatedly observed, due to the greater minor allele frequency [30, 32-34].
