also emphasized that very dense SNP maps provide the greatest increment in linkage information when parental genotypes are unavailable. Due to the characteristics of our particular sample, which is rich in small nuclear families (in most cases including no more than one parent), the increment in information achieved is of particularly great value, and it is doubtful that we would have identified a statistically-significant linkage in this sample without a highdensity marker set because of the relatively small set of informative families. The fact that AD is less common in AAs than in EAs suggests that a higher genetic loading is required in this population for the trait to be expressed, which may also have worked in our favor to identify a significant linkage. It is difficult to estimate power for a study such as this; power depends on several factors of which only two (sample size and marker density) are known. Other factors include heterogeneity and effect size. Risch and others have published power calculations for affected sibling pair linkage based on sample size and λ. Based on Risch’s power calculations (42) assuming 100 ASPs (approximately the number included here) and close linkage, we had sufficient power to detect
