Figure 2.5 The structure of two MS1 (locus D1S7) VNTR alleles (Berg et al., 2003) . The alleles are both relatively short containing 104 and 134 repeats – alleles at this locus can contain over 2000 repeats. The alleles are composed of several different variants of the 9 bp core repeat – this is a common feature of VNTR alleles
Two important categories of tandem repeat have been used widely in forensic genetics: minisatellites, also referred to as variable number tandem repeats (VNTRs); and mi- crosatellites, also referred to as short tandem repeats (STRs). The general structure of VNTRs and STRs is the same (Figures 2.5 and 2.6). Variation between different alleles is caused by a difference in the number of repeat units that results in alleles that are of different lengths and it is for this reason that tandem repeat polymorphisms are known as length polymorphisms.
Variable number tandem repeats – VNTRs
VNTRs are located predominantly in the subtelomeric regions of chromosomes and have a core repeat sequence that ranges in size from 6 to100 bp [17, 18]. The core repeats are represented in some alleles thousands of times; the variation in repeat number creates alleles that range in size from 500 bp to over 30 kb (Figure 2.5). The numberofpotentialallelescanbeverylarge:theMS1locusforexample,hasarelatively short and simple core repeat unit of 9 bp with alleles that range from approximately 1 kb to over 20 kb – which means that there are potentially over 2000 different alleles at this locus . VNTRs were the first polymorphisms used in DNA profiling and they were suc- cessfully used in forensic casework for several years . The use of VNTRs was, however, limited by the type of sample that could be successfully analysed because a large amount of high molecular weight DNA was required. Interpreting VNTR profiles could also be problematic. Their use in forensic genetics has now been replaced by short tandem repeats (STRs).]
Short tandem repeats – STRs
STRs are currently the most commonly analysed genetic polymorphism in forensic genetics. They were introduced into casework in the mid-1990s and are now the main