Figure 1.

Symptoms of mood disorders are shared among common complex disorders, rare chromosomal disorders, and monogenic Mendelian disorders. Although many common complex disorders and rare Mendelian disorders share psychiatric symptoms, they do not always share the same genetic risk factors. Common complex disorders, such as bipolar disorder, are likely influenced by many genetic variants with small effect, in addition to environmental risk factors. Rare chromosomal disorders are characterized by large chromosomal deletions and duplications, which could potentially affect hundreds of genes. Rare monogenic Mendelian disorders are caused by characteristic mutations in a single gene. These differences in genetic risk factors have important consequences for risk prediction, genetic testing, and counseling.