In all of the studies discussed: twin, candidate gene association, and genome-wide association, we will address two types of genetic polymorphisms: single nucleotide polymorphisms (SNPs) and variable number tandem repeats (VNTRs). SNPs are sites at which a single nucleotide differs among individuals within a population. SNPs can either occur in the coding sequence of a gene and thus alter amino acid sequence (termed non-synonymous) or, more commonly, they may be outside the coding sequence and alter gene regulation. In both cases, a SNP may alter a biological function itself (coding or gene expression), or be linked to another polymorphism that is functionally significant. VNTRs are polymorphisms in which a variable number of short repetitive sequences (tandem repeats) are present at a given locus; as with SNPs they may have direct functional consequences or may be linked to some other functionally significant polymorphism.
