DNA and the law

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EDITORIALS

DNA and the law The speed of the legal process is often likened to that of a tortoise, yet there are times when the law trips in its endeavours to keep abreast of scientific advance. The rapidity with which American courts adopted DNA fingerprinting was not without cost. This technique, discoveredl and developed2 by Prof A. Jeffreys, is increasingly in demand. DNA fingerprinting has been used in the courts, especially in Britain and the USA, in several hundred criminal cases and several thousand paternity determinations, and in West Germany in 1989 for the first time as proof in law. The value of the DNA fingerprint, like that of the dermatoglyphic print, lies in the extremely low probability of duplication. Recent American experiences indicating that there may be weaknesses in the application of this test3 are disturbing because they have cast doubt on the validity of an extremely

powerful technique. A DNA fingerprint defines a characteristic pattern of pieces of genetic material for each person, very similar to the bar code on items in supermarkets, in which bands of different thicknesses are separated by spaces of different widths. In the DNA pattern the bands represent DNA fragments separated by size. When sufficient numbers of such DNA fragments in two unrelated individuals are compared, the probability that they show the same pattern may reach less than 1 in 1 milliard. If a fingerprint of DNA derived from a biological trace left at the scene of a crime corresponds to that of a suspect, there is a very high probability that it was he who left the trace. Nevertheless, mistaken judgments may be made if the test is not done correctly, and in the USA there was a Congressional inquiry in March, 1989, into its use. There are four sources of difficulty-the potential for human or technical error in sample treatment or data analysis; interpretation of DNA patterns;

uniformity of criteria used to determine whether two patterns match; and lack of population studies on which the predicted likelihood of a mismatch is based. Two years ago in the USA a blind trial was carried out by the California Association of Crime Laboratory Directors, in which fifty specimens were sent to the three commercial US laboratories. Although the laboratories knew that they were participating in a quality control experiment, two of them reported an incorrect result. In both cases fragment patterns were erroneously declared to be identical. In one case the mistake was due to non-binding of DNA to a filter; in the other, two samples had been interchanged. In a murder case dealt with in the Bronx, Dr Eric Lander of New York was among the molecular biologists called as expert witnesses. He was worried that two dissimilar fragments in the genetic fingerprint offered as evidence had, without proof, been irresponsibly dismissed as impurities. He further questioned whether a sex determination test carried out with a male-specific probe could identify a blood stain as coming from a woman. Reliability of the analysis was criticised by Dr Howard Cooke of the Medical Research Council Human Genetics Unit in Edinburgh, who had developed two of the probes that were used. In the pattern of bands representing DNA fragments separated in this test according to size, some may be so close to each other that minor deviations in position are barely recognisable, while variants in the population often produce fragments of similar sizes. He questioned whether the samples in this instance could be matched with certainty by eye. A curious feature of this case was that, after the preliminary hearing, the defence and prosecution expert witnesses held discussions among themselves and decided that some of the evidence was not reliable.4 The prosecution nevertheless retained the evidence in its submissions. The

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judge ruled that, although the generally reliable, in that particular

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the results were not-the first time apparently that any judge had excluded DNA evidence of a match 5 on grounds of insufficient reliability. In a case of sexual molestation of a five-year-old girl in Portland, Maine, the prosecution dramatically withdrew its DNA fingerprint evidence that a semen sample on a paper tissue found at the scene of the crime matched a blood specimen from the defendant. Again the criticism was the method of matching. The two gels could not be superimposed because, as so often happens, the bands on one had migrated more rapidly than those on the other. The laboratory had therefore used a correction factor based on the migration of a known control sample to adjust one band. Then undisclosed evidence-a piece of paper sent by mistake to the defence among other paperswas revealed showing that another control had given a completely different correction factor.6 In a rape/murder case in Texas, the suspect was convicted and sentenced to death on the evidence of a matching pattern. The odds against this pattern occurring by chance were stated to be 96 million to 1 but took no account of the fact that the crime occurred in a small inbred town whose population derived from only a few founders. Little is known about the frequencies of the DNA fragments of different sizes, or about unpredictable concordances in certain populations. Years of work in human genetics have shown how populations differ in gene frequency, and therefore in the frequency of the responsible DNA sequences, and it is certain that the fragments in the fingerprints will also differ in frequency. case

Many authorities, uneasy about relying on identification made by commercial firms, are installing molecular genetic divisions in their forensic institutes. In West Germany such divisions already exist in the Federal Crime Office in Wiesbaden, and have been established by the police in Berlin and Stuttgart. In the UK, the Home Office forensic laboratories (eg, in Wetherby) have facilities for DNA studies. The Blood Tests (Evidence of Paternity Amendment) Regulations 1989 allowed the inclusion of DNA testing by testers approved by the Home Office. In September, the Director of Justice for New York State announced an attempt to regulate the forensic use of the method. Three state-run laboratories were established but private laboratories were permitted if they were accredited by the state after independent validation of the methods used, had

polymorphisms (eg, blood groups, serum proteins, enzymes) incorporating a sufficient number of polymorphic systems can exclude paternity virtually without doubt; if a man is not excluded, the probability that he is the father is often as high as several thousand to one. Compared with conventional levels of significance used in scientific work, these levels are high enough to be convincing, and there is nothing to be gained by seeking the apparently higher figures claimed for DNA. Moreover the conventional systems relate to features that courts can understand. But the widespread media coverage of DNA fingerprinting has encouraged many courts and many of the individuals concerned to request DNA analysis to the exclusion of the older techniques. Magistrates are ordinary intelligent people who can understand simple blood groups and similar serological systems, the concept of the gene responsible being at a particular point on a chromosome, and the way a gene is transmitted from parent to child. Although they can also see a band on a DNA fingerprint, it is more difficult for them to comprehend the limitations of the technique; where a particular band of DNA is located on the chromosome and what it does; and the nature of the proof that a certain fragment of DNA of a given size in two people represents the same piece of DNA from the same chromosome site. At the Congressional hearing on DNA fingerprinting last March, the chairman called for licensing and regular proficiency testing of private and government laboratories, and Congress seems likely to introduce regulatory legislation. The high standards of such work in Britain, the insistence of Cellmark Diagnostics that those using its probes show themselves competent before being provided with them, and similar safeguards make it unlikely that such legislation will be required in the UK. Nevertheless, lawyers in Britain and elsewhere will doubtless learn the lesson from these American cases-this type of evidence can be challenged on several fronts. 1.

Jeffreys AJ, Wilson V, Thein SL. Hypervariable minisatellite regions in human DNA. Nature

1985; 314: 67-73.

Jeffreys AJ, Wilson V, Neumann R, Keyte J. Amplification of human minisatellites by the polymerase chain reaction: towards DNA fingerprinting of single cells. Nucleic Acids Res 1988; 26: 953-71. 3. Anon. DNA fingerprinting hits a snag. Lancet 1989; i: 1403. 4. Lander ES. DNA fingerprinting on trial. Nature 1989; 339: 501-05. 5. Anderson A. Judge backs technique. Nature 1989; 340: 582. 6. Anderson A. DNA fingerprinting on trial. Nature 1989; 342: 844.

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participated in state proficiency testing programmes, and were approved by a scientific review board. There is no doubt about the advantage of DNA analysis in identifying the perpetrators of a serious

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crime. Minute amounts of DNA extracted from a biological substance (semen, blood), often of considerable age, can produce a result which would be impossible by other methods. Investigations of disputed paternity are an entirely different matter. Long-established methods for identifying serological

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