This document discusses mapping complex traits related to common disorders through genome-wide association studies (GWAS). It notes that while Mendelian disorders are rare and caused by single genes, common disorders are influenced by multiple genetic and environmental factors and cluster in families. The document provides examples of GWAS studies that have identified genetic variants associated with atherosclerosis, ischemic stroke, and intracerebral hemorrhage. It distinguishes between single-gene disorders and complex disorders that have an interaction of genetic and environmental factors.