A major theme in prioritizing brain phenotypes for large-scale genetic analyses is the presence of significant heritability, indicating that a proportion of individual variance in the phenotypes can be explained by genetic variation. The effect size of individual variants cannot be inferred from the heritability and higher heritability does not translate to a higher likelihood of a positive GWAS finding. However, phenotypes whose heritability is not significantly different from zero may not be good candidates for GWAS analysis because of the lack of variance due to additive genetic factors. Jahanshad et al. (2013a) screened a number of regions of interest across multiple cohorts, finding high heritability for most major white matter pathways and consistent heritability across pedigree-based and twin-based samples (Fig. 4; Kochunov et al. 2012; Jahanshad et al. 2013a). A variety of DTI parameters could be measured reliably among individuals, and genetic factors explained a substantial proportion of the variance for the most commonly used DTI measures.
