Genome-wide association (GWA) data also provide an opportunity to investigate whether traits are influenced by overlapping genetic risk factors. On the basis of GWA results for one trait, for instance, neuroticism, performed in one sample (the discovery sample), a polygenic score is calculated for each individual in another sample (the target sample). These polygenic scores are obtained by taking a set of top single-nucleotide polymorphisms (SNPs), for example, all SNPs with P-values below 0.1 and multiplying the individual's genotypic score (0, 1 or 2) by the effect of the SNP. If the polygenic scores are significantly related to a second trait, for instance, MDD, in the target sample, this indicates that the two traits, namely neuroticism and MDD, are influenced by overlapping genetic risk factors. In this manner, a genetic relationship was observed for schizophrenia and BD23 and for MDD and anxiety.24 In the former study, polygenic scores based on a GWA study in schizophrenia explained between 1 and 2% in BD. In the latter study, polygenic scores based on a MDD GWA study explained ∼2% of the variance in anxiety disorders.
