All analyses were performed separately in COGA AA (COGA-AA) and EA (COGA-EA) subsamples, then meta-analyzed (COGA-AA+EA). Due to the complex pedigree structure in COGA, methods using genomewide genotype data such as Genomewide Complex Traits Analysis (Yang et al., 2011) could not be used to estimate heritability or genetic correlations. Instead, using an approach similar to those applied in classical twin studies, the Sequential Oligogenic Linkage Analysis Routines (SOLAR8.3.1) (Almasy and Blangero, 1998) package, which was designed to partition variance in large pedigrees, was used to estimate heritability of SRE scores (i.e., proportion of total phenotypic variance attributable to latent genetic influences), as well as the genetic correlations between the SRE measures and AD and DSM-IV AD criterion count (Almasy and Blangero, 1998). Sex and birth cohorts (birth year: 1890-1929; 1930-1949; 1950-1969; >=1970) were important covariates as shown in previous studies (Grucza et al., 2008); therefore both of them and the first four PCs were used as covariates in SOLAR analyses.
