Tandem repeat polymorphisms
Tandem repeat polymorphisms are caused by differences in the number of copies of a DNA sequence which is repeated at a certain site of the chromosome. This can occur on any chromosome and the repeating sequences tend to be 15-100 base pairs long and can be found both in and between genes. Clusters of these repeating regions are called minisatellites. Restriction endonucleases can cleave at the site of a tandem repeat and the size of the fragment produced is dependent upon the number of repeats it has. PCR, with primers which flank the tandem repeat, can be used to amplify duplex DNA and produce DNA fragments which increase in size as the number of tandem repeats increases.
One type of tandem repeat polymorphism is a simple sequence repeat (SSR) - a repeat of 2-9 nucleotides. These are common in the human genome, approximately 1.5 million of them, and are considered greatly polymorphic in human populations – this property making them useful as genetic markers. Variable number tandem repeats (VNTRs) are longer than SSRs with a repeating unit of approximately 10-60 nucleotides. VNTRs are able to be used in DNA fingerprinting because larger restriction fragments are produced making them more easily identifiable in gel electrophoresis.