The DNA instructions are passed with very great fidelity from one generation to the next. However, occasionally a mutation occurs and effectively changes one base to another. The site of this base change is called a single nucleotide polymorphism (or SNP). SNP variation occurs when a single nucleotide, such as an A, replaces one of the other three-nucleotide letters—C, G, or T.
On average, SNPs occur in the human population more than 1 percent of the time. Because only about 3 to 5 percent of a person’s DNA sequence codes for the production of proteins, most SNPs are found outside of these protein coding regions. SNPs found within a coding sequence are of particular interest as they are more likely to alter the biological function of a protein. Although many SNPs do not produce detectable changes in protein function, other SNPs may predispose people to disease and influence their response to drug regimens.
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