DNA profiling and the forensic use of DNA evidence have undergone considerable development since the Australian Institute of Criminology first examined this topic in 1990 in Trends and Issues no. 26. Some of the laboratory techniques described in that report have since been refined so that more precise DNA profiling is now possible, and a greater range of criminal investigations can benefit from the use of such forensic techniques. Moreover, the proposal in that report for a national DNA database has now been advanced, with the establishment on 1 July 2000 of the CrimTrac agency. However, many of the issues raised in relation to scientific reliability, standardisation of profiling techniques, laboratory accreditation and quality control, improved population and data analysis, and privacy are still the subject of disputation in legal proceedings.
This paper examines the science of DNA identification and its use during criminal investigations and in criminal proceedings, including criminal trials, appeals and post-conviction proceedings. It describes the main benefits and costs of the increasing role of DNA identification in the criminal justice system.
The Science of DNA Identification
A Natural Identifier
Deoxyribonucleic acid (DNA) is a long molecule, found in the cellular nuclei of living organisms. Since 1954, scientists have recognised that the chemical structure of an individual's DNA encodes information about that individual's inherited characteristics. The present limits on genetic science mean that a direct analysis of a person's DNA will yield only limited information about individual characteristics, although some research suggests that investigators may in the future be able to discern specific physical traits such as hair, eye and skin colour from forensic samples (National Institute of Justice 2000, pp. 18-19; van Oorschot et al. 2001). Rather, the current utility of DNA analysis to the criminal justice system arises from the comparison of DNA from two sources, such as DNA from a crime scene and DNA from a suspect, to determine the relationship between those sources.
Traditionally, the identification of a person has required the observation of that person's entire body or of localised special characteristics such as fingerprints, blood group or hair type. By contrast, DNA analysis allows identification by reference to the information contained in any human nucleic cell, irrespective of which part of the body the cell comes from. The DNA in a human cell is unique, the product of sexual reproduction that combines half of the mother's DNA and half of the father's DNA. Every cell in an individual's body is the result of cellular division, which copies the DNA in the newly fertilised cell into every other nucleic cell. As a result, DNA in a cellular nucleus is identical throughout a human body but variable between any two humans, making it a natural alternative to artificial human identifiers, such as names or tax-file numbers. The notable exception is identical twins, who develop from a single fertilised cell and hence have identical nuclear DNA.
The technique of "DNA identification" compares the DNA of two bodily samples to ascertain whether or not they came from the same human being. Identity of DNA in the cells across both samples implies that the samples are derived from the same person (or identical twins); non-identity implies different human sources. Alternative comparative techniques can be used to determine whether or not there is a familial relationship between the two human sources. For example, a matrilineal relationship can be inferred from a comparison of DNA in mitochondria, which pass from mother to child unchanged by sexual reproduction.
Comparison of human DNA molecules does not require analysis of the entire DNA molecule, as about 99.9 per cent of DNA is common to all people. Rather, DNA comparison need only focus on a portion of the remaining 0. …