Table 1.

Variable number tandem repeats

A variable number tandem repeat occurs in DNA when a pattern of one or more nucleotides is repeated, and the repetitions are directly adjacent to each other. In the human PER3 gene, the short allele contains four tandem 54-bp repeats, and the long allele has five such repeats. As each individual has two sets of alleles, this produces three possible genotypes: PER3 4/4, 4/5 and 5/5.