Ever since the first human 'DNA fingerprint' was made by hybridization of multi-locus probes to variable number tandem repeats (VNTRs) (Jeffreys et al., 1985) and the first DNA evidence was presented at court (Gill et al., 1985), tandem repeat sequences have been the favoured targets for forensic DNA analyses. Today, short tandem repeats (STRs) with four nucleotide repeat units are preferred, mainly because they are easily amplified in polymerase chain reactions (PCRs) and because they are shorter than VNTRs and more likely to be intact in low-quality samples often recovered from crime scenes. Thirteen STRs located on different chromosomes were selected for the Combined DNA Indexing System (CODIS) database (Budowle et al., 1999), and today there are several commercial products available that allow amplification of all CODIS loci in one multiplex PCR. These kits are validated and used by forensic laboratories all over the world, and they have facilitated the development of standardized databases, which have proved to be highly valuable tools for national and international law enforcement. Each CODIS locus has many different alleles because mutations happen frequently in tandem repeat sequences (on average 1 mutation per 300 generations), and therefore it is highly likely that different alleles are found in different individuals, which makes tandem repeat sequences very suitable for identification purposes.

Traditionally, the main purpose of forensic genetic investigations has been the identification of human remains found at crime scenes and mass disasters, or the determination of family relations where the available information is

Molecular Forensics. Edited by Ralph Rapley and David Whitehouse Copyright 2007 by John Wiley & Sons, Ltd.

disputed or uncertain (e.g. in paternity or immigration cases). However, with the increase in the number of DNA profiles in national and international databases of criminal offenders, DNA profiling has become more and more important as an investigative tool for the police. This trend will most likely continue as more knowledge of the human genome is disclosed and new forensic genetic tools are developed. In this chapter, the present and potential applications of single nucleotide polymorphisms (SNPs) are discussed.

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