The genetic code is composed of nucleotide base pairs (bps) that are organized into genes. Genes represent segments of the genome that contribute to particular phenotypes or functions. Alternative forms of genes, termed “alleles,” can be identified by examining specific locations within the gene. One type of genetic variation, produced by repeated sets of base pairs, is referred to as a Variable Number Tandem Repeat (VNTR). Both DRD4 and 5HTT contain well-characterized VNTRs. VNTRs may alter the product of the gene (as with DRD4) if they occur in the coding region (i.e. the Exon), or they may influence the amount of product produced (as with 5HTT) if they occur in a promoter region. Within the coding region every three base pairs codes for a particular amino acid.
