singletons, copying genotypes from one MZ twin to the other will introduce errors due to somatic mutations (Poduri, Evrony, Cai, & Walsh, 2013), which are present only in one MZ twin and not the other. If we assume that each person carries 50 somatic SNP mutations in their genome (Neale et al., 2012), and 3.5 million SNPs (1000 Genomes Project Consortium, 2012), somatic mutations will represent a genotype error rate of approximately 50/3,500,000 = .001% in the MZ twins only. In the context of the present study, we suggest that the increase in power obtained by adding 381 individuals outweighs the increased genotype error expected from somatic mutations. Furthermore, we do not evaluate singletons in our single variant tests. Therefore, somatic mutations in the present study will only affect gene-based tests in the exome, where we expect less than one somatic exonic mutation per individual.
