The initial discovery of DNA-based methods for human individuation was made by Alec Jeffreys in 1985. It was first used to support a criminal investigation a year later when Jeffrey’s original techniques were used to compare DNA profiled from semen recovered from the bodies of Linda Mann and Dawn Ashworth who had been raped and murdered, in 1983 and 1986, respectively. Since then a series of technological innovations has meant that DNA profiling (known also as DNA fingerprinting and DNA typing), based on polymerase chain reaction amplifications of a varying number of short tandem repeat loci found at different locations on the human genome, has become the ‘gold standard for identification’ in a range of contexts. It has also been described as the most significant advance in forensic science support to policing since the introduction of fingerprinting more than 100 years ago.
DNA extraction methods have now advanced to the point that profiles can be obtained from very small numbers of human biological cells recovered from scenes of crime (in, for example, blood, semen, saliva, hair roots and scalp detritus, flesh, skin, vaginal fluids and nasal secretions). It is also increasingly possible to distinguish multiple contributors to mixed biological samples and thus more easily to identify possible perpetrators of crimes in which genetic material from both victims and offenders is co-mingled.
While the initial uses of DNA profiling were largely confined to reactive forensic casework, the subsequent ability to construct digital representations of profiles and store them in continuously searchable computerized databases has made possible a vastly expanded role for DNA profiling in many criminal investigations. An increasing number of criminal jurisdictions have established such databases, which include both profiles obtained from crime scenes and those obtained from individuals during the course of criminal investigations.
The largest such database, containing more than 4 million subject sample profiles (comprising more than 5 per cent of the population and almost one quarter of a million crime scene profiles), is the National DNA Database of England and Wales (NDNAD). Current legislation (the Criminal Justice Act 2003) empowers the police to take and retain samples from all those arrested on suspicion of involvement in a recordable offence and to retain samples regardless of whether the individual in question is subsequently charged or convicted of the offence in question. Records contained on the NDNAD and referred to as SGM+TM profiles comprise measurements of 11 genetic loci, each of which is on a different chromosome. It is currently calculated that the chances of two unrelated individuals sharing the same measurements for this set of loci are less than one in a billion.
Patterns of crime scene attendance and the availability and potential significance of biological evidence mean that DNA profiling is used in less than 1 per cent of criminal investigations. However, in cases when DNA is recovered from scenes of crime, detection rates are markedly improved – e.g. in 2004–5 the overall detection rate rose from 25 per cent to 40 per cent when DNA profiles were recovered, and the domestic burglary detection rate rose from 16 per cent to 41 per cent. It is generally agreed that an understanding of exactly how the availability of DNA impacts on the investigative process needs more detailed research in order that its full potential is maximized.
There is increasing interest in the genetic analysis of biological material found at crime scenes in order to infer the physical characteristics of the person who left such material. The most advanced of such methods utilize arrays of single nucleotide polymorphisms to characterize the ‘biogeographic ancestry’ of individuals, but there are other efforts to analyse genetic data capable of predicting eye and hair colour, as well as height and other phenotypical attributes.
Crime scene examiners (CSEs); Evidence; Fingerprints; Forensic and investigative psychology; Forensic investigation; Offender profiling.
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