Given that we found some evidence for association between GABRA2 and quantitative measures of DSM-IIIR and DSM-IV AD in our sample, we expected to detect significant associations with the diagnostic DSM-IIIR and DSM-IV AD phenotypes. However, only a trend towards association (p < 0.1) with DSM-IIIR dependence was observed. The low levels/absence of association for the binary measures of AD may be due to several factors. First, the binary trait (within-family) association analyses excluded both MZ families and families with only one twin available. The reduction in the number of informative (phenotyped) individuals included in our analyses resulted in an overall loss in power. Second, our community-based twin sample is not as severely affected by AD as other samples that have shown evidence for association between GABRA2 and AD, such as the COGA sample with multiple affected family members. Therefore, our sample may have a lower genetic load with respect to AD. Correspondingly, when Dick et al. (2006) compared the onset of AD across the lifespan in a combined sample of COGA families and control families with that of a
