DNA Evidence

DNA EVIDENCE
This is based on information extracted from chapter 20 of the book "Crime Busting" by Jenny Ward.

Although it is not disputed that DNA finger-printing is the biggest forensic breakthrough of all time, each aspect of the process, nevertheless, is weighed down with difficulties. The sample, the tests, the interpretation of the tests, and the presentation of the evidence in court, have all come under fire.

When a sample of blood is drawn for, say, a paternity test, the investigator can take as much as is needed to produce an unequivocal result. In a criminal case, a sample taken from the victim or the scene of crime may contain elements of DNA from both the victim and the perpetrator, or it may be contaminated by other biological matter at the scene. The sample may also be very small, or have deteriorated over time or by heat. Once in the laboratory, there is always the possibility of contamination, despite precautions, by mishandling of reagents, samples or equipment. DNA labs, and their scientists, have to be scrupulously clean.

Contamination is also possible not just in the environment of the laboratory, but during the actual testing, especially if the case sample and the suspect sample are run side by side. It is safer to run a control sample between the two, as is now done. Another problem is that however carefully samples are handled, there is always a possibility that they can get mixed up, and at least one such case has already been reported. Since DNA was first used, different methods have evolved; so it isn't always possible to compare samples across time, or from different labs using a different technique. (Prosecution and defence invariably use different laboratories.)

There are other difficulties with comparison. What criteria do you use to decide that two profiles are identical? In physical fingerprinting, a rule gradually evolved in the UK that there had to be sixteen points of resemblance for the fingerprint to count as identical. How many bands have to match for a DNA profile to count as identical? If one DNA band is very faint compared with another at the same position, is that a match? Sometimes the DNA material rises up the gel more slowly in one place than another, so that the pattern may be identical but one profile has shifted to be slightly out of alignment. Is this identical? What if one band is slightly thicker than another? The operator needs a great deal of unbiased skill and judgement to make these decisions.

Retesting is not always possible if the case sample is very small. A lot of debate has focused on statistical probabilities. What are the chances that another person, and not the suspect, has generated the sample that was the basis of the test? The normal way that the very high probability figures quoted in DNA cases are worked out is by calculation on the basis of the product rule. According to this, the probability that a number of matches will occur simultaneously is determined by multiplying the probability that each case will occur. To give an example: in 1985 Mec Jeffreys (now Sir Mec) tested twenty Caucasians and estimated the probability of a match as 1 in 30,000 million. This was how Galton worked Out his statistical probability of 1 in 64,000 million for finger-prints. And like fingerprints, DNA bands have the same problem.

The maths only works (if it works at all) if the bands, or individual features, are independent of one another. If there are some bands which always occur together, then shorter odds result. It takes a leap of faith to go from twenty Caucasians to 1 in 30,000 million, and more recent calculations have been more plausible.

A further problem with scientific evidence is that the samples are getting smaller and smaller. Multi-locus probe testing needed about 50,000 sperm to obtain a good DNA profile, whereas single-locus probe testing needs only 20,000. There is also a replication technique called polymerase chain reaction (PCR), which replicates small amounts of DNA until the sample is large enough to be tested. This needs about 500 sperm. If the sample is contaminated, by the way, the contamination is also replicated.

In July 1997, The Times reported that a new advance had allowed DNA to be extracted from a single hair found at the scene of the murder in Chillenden, Kent, in 1996, when Lyn Russell and her 6-year-old daughter Megan were battered to death by a stranger. This was mitochondrial DNA, taken from a strand of hair, rather than its root. If someone is imprisoned for this crime on a match from a single strand of hair, used for the first time in court, and this is the only unconfirmed piece of evidence against the accused, will it be a safe conviction?

The problems of forensic science and its practitioners are inherent in the system. There will always be partisan witnesses and bad science in court. In theory, the criminal justice system should forestall a poor case from being presented. If it comes to court, the defence should be as able as the prosecution, with the same resources; the evidence and the judge's summing up should be presented fairly, and even if they are not, the jury should act as a safety net and acquit. If these safeguards fail, the only recourse is to use the weight of public opinion to fight for justice, to arouse controversy.

Posted: August 2000

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