We used the SNPs selected from GCTA-COJO analysis to create a genetic predictor (also called “genetic profile score”) for each of all the full sibs using PLINK25. We then adjusted the genetic predictor by the first 20 principal components (PCs) generated from the principal component analysis (PCA)26. By comparing the predictors within and between families, we partitioned the variance in the predictor analysis into components due to real SNP effects (Vg), errors in estimating SNP effects (Ve), and population structure (Cg + Ce), as described in the Online Methods below.
