Individual variability results from a complex interaction of genetic and environmental factors. However, linkage disequilibrium, genome-wide association, case-controlled, and family-controlled studies have demonstrated the heritability of complex behaviors and response to drug treatments, suggesting that specific genes or alterations in genes may be responsible for the differences in behavior. Common genetic variations among individuals include single nucleotide polymorphisms (SNPs), in which a single nucleotide of the genome is altered. SNPs occur every 100-300 base pairs and account for approximately 90% of human genetic variation. The nature of the change produced by the SNP greatly depends on which nucleotide is being altered and where this change occurs in the gene. For instance, synonymous SNPs will alter the nucleotide without changing the resulting amino acid (also called a “silent mutation”). Non-synonymous SNPs are produced when the nucleotide substitution alters the resulting amino acid. Additionally, these alterations can occur in promotor, exonic, or intergenic regions and, consequently, may differentially affect transcription, processing, stability, translation, folding, transportation, and ultimately, function of the corresponding gene product.
