Power is always a concern for genetic linkage studies of complex traits such as AD. Phenotypic ascertainment is time consuming and expensive, collecting families with multiple affected individuals is challenging, and extending sibling pairs to include additional siblings and parents, difficult. Thus, methods that increase the power of linkage studies with such samples provide an efficient means of extracting the largest possible amount of linkage data. Considerably more linkage information may be extracted from a linkage sample using a high-density SNP map, compared to what is now considered a low-density (400 marker) short tandem repeat (STR) map (such as that employed in our previous linkage studies with parts of this sample). The information increment was estimated to be close to 75% in families where parental genotypes were unavailable but <50% where parental information was available (40). Evans and Cardon (41) 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
