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 reduced sample composed of control families alone, they observed less significant allelic effects among the controls (p = 0.013 vs 0.0035). Nevertheless, heritability for AD is high (47−75%) and the sibling relative risk is approximately 2 within our sample (Heath et al., 1997).
