approach. Before we discuss GWAS, it is worth noting a distinction between narrow and broad-sense heritability: broad-sense heritability is the proportion of variation in a phenotype (here, individual variations in brain measures) that can be explained by genetic effects. These effects may include dominance and epistasis—interactions between SNPs or genes in different parts of the genome. The narrow-sense heritability is the proportion of variance in a brain measure that is accounted for by additive genetic factors (and this is typically a smaller proportion of the trait variance). These additive genetic effects are the types of statistical effects that GWAS aims to detect.
