Polymorphisms

Repetitive genetic elements are an important class of polymorphic DNA. These sequences consist of several repeats of a simple DNA sequence pattern, and they typically do not encode a protein or have strict requirements of size and sequence. For example, the two base pairs cytosine (C) and ade-nine (A) may be found together multiple times, resulting in a "CACACACA" sequence. If another copy of this sequence were found as "CACA" (two CA pairs shorter), then this sequence would be polymorphic. Repetitive genetic elements include microsatellites or STRs (short tandem repeats) and the minisatellites or VNTRs (variable number of tandem repeats), which are distinguished primarily on the basis of size and repeat pattern: The repeated sequence in microsatellites range from two to six bases, while in a VNTR it ranges from eleven to sixty base pairs.

Differences in single base pairs, known as single nucleotide polymorphisms (SNPs), are a valuable class of polymorphism that can be detected by DNA sequencing, RFLP analysis, and other methods such as allele-specific PCR and allele-specific DNA hybridization. Many RFLPs are due to single nucleotide polymorphisms. There are hundreds of thousands of SNP loci throughout the human genome, making them especially valuable for mapping human disease genes.