Long-range phasing and genotype imputation Long-range phasing of all chip-genotyped individuals was performed with methods described previously,21 and imputed into chip-typed individuals and their close relatives using methods21 based on IMPUTE.28 In brief, phasing is achieved using an iterative algorithm, which phases a single proband at a time given the available phasing information about everyone else that shares a long haplotype identically by state with the proband. Given the large fraction of the Icelandic population that has been chip-typed, accurate long-range phasing is available genome wide for all chip-typed Icelanders. The informativeness of genotype imputation was estimated by the ratio of the variance of imputed expected allele counts and the variance of the actual allele counts:
