Law in miniature

DNA evidence has played an important part in the criminal justice system for over 20 years, but the development of techniques for testing sub-optimal biological samples, known as low template (LT) DNA, has caused controversy. The Court of Appeal judgment in the conjoined appeal in R v Reed and Reed and Garmson provided guidance with regard to the application of scientific techniques but there remain grounds upon which the results of DNA profiling can still be successfully challenged, as long as part 33.3 of the Criminal Procedure Rules 2010 is observed.

Standard DNA profiling can be done on one billionth of a gram of DNA. However, as the techniques of manipulating DNA have developed it has become possible, in certain circumstances, for profiles to be produced from much smaller quantities than this. At a crime scene, there are broadly two types of biological material from which DNA can be extracted: particulate items of material, such as bone, hair and stains of biological fluids, such as blood and sweat.

The genetic information contained in DNA is unique to each individual and is the same in all of an individual's bodily fluids and tissues. There are ten 'marker' regions of DNA on the chromosomes that are known to differ from one person to another and it is by comparing samples' marker regions that identifying a match is sought. The relationship between different profiles is expressed as 'match probability'. However, where fewer than these ten regions of DNA have been successfully targeted, there will be an incomplete or 'partial' profile and the match probability can be significantly affected.

Mix ups

DNA degrades over time and certain portions (known as alleles) degrade faster than others, so the quality of the scientific interpretation remains of central importance.

Another factor affecting the integrity of DNA sampling arises in the case of 'mixed samples', where samples recovered may contain DNA from more than one person.

Difficulties can also arise when it comes to trying to work out how the accused's DNA came to be found at the crime scene, as there are a number of possible methods of transmission that need to be considered:

• Primary transfer '“ where DNA is transferred because of direct contact between the person it belongs to and the item it is recovered from.
• Secondary transfer '“ where DNA is transferred via contact with another person who then deposits it at the crime scene.
• Tertiary transfer '“ where DNA is transferred onto an object, which is then handled by someone else who then, again, deposits the DNA at the crime scene.

Other complicating factors include where the perpetrator is a close relative of the suspect or that the suspect has an identical twin. In terms of interpretation of data, most of the statistics on which match probability calculations are founded are based on ethnicity and relatedness, so that a match probability of one in one million could easily change to a match probability of one in five or ten million, if the statistics in question are re-examined so as to exclude certain ethnic groups.

Finally the issue of storage, reservation and contamination of the samples is of central importance and the audit trail of the sample is one that should be carefully examined.

Samples of biological material or fluid containing less than one nanogram or 1,000 picograms of DNA, or those of poor quality, will be classed as low template DNA. This evidence is obtained using a profiling technique called low copy number DNA. The need to use low template DNA typically arises where the available crime scene sample is very small, and/or where the sample has been collected from a surface the suspect may have touched, such as a door handle. Clearly, the smaller the sample tested the greater the chance of inaccuracies, through statistically random (stochastic) effects.

The currently accepted threshold below which the magnitude of stochastic effects may be expected to cause difficulties in interpretation is somewhere between 100 to 200 picograms.

The possibility of inaccurate or incomplete results led many to question the use of low template DNA in criminal trials. Following specific concerns raised in of the case of R v Hoey (Sean) [2007] NICC 49, it seemed for a while that evidence based on these sub-optimal biological samples might no longer be admissible.

Caddy report

Hoey was charged with murder and various other offences in connection with the 1998 Omagh bombing. Much of the case against him was based on low copy number DNA evidence. Concerns by the presiding judge led to a review by the CPS in 2008 and the Caddy report, a broader-ranging review of low template DNA, commissioned by the Forensic Science Regulator in 2008.

The outcome of both reviews supported the science and techniques. However, the Caddy report highlighted a number of caveats when dealing with low template DNA with regard to the use of appropriate experts and the guidance given by the presiding judge to a jury. In particular, that:

• the nature of the original starting material is often unknown;
• the time at which the DNA was transferred cannot be inferred;
• the opportunity for secondary transfer is increased in comparison to standard DNA profiling;
• when DNA profiles match as a result of low template DNA analysis, the significance of the match can only be reported on the probability that the two DNA profiles match; and
• where results have been obtained from low template DNA analysis it is inappropriate to comment upon the cellular material from which the DNA arose or the activity by which the DNA was transferred.

Finally, the Caddy report recommended that all samples of biological material or stains of biological fluid being submitted to DNA testing should be quantified, so that it is clear at the outset whether the amount of available material makes the sample suitable for standard DNA testing or low template testing, and, if suitable, that the likelihood of any stochastics effects are properly assessed.

Despite the endorsement of low template DNA by the CPS review and the Caddy report, there remained a sense of unease about its credibility. For the most part, this has been addressed by the Court of Appeal decision in the conjoined Reed and Garmson appeals.

In Reed, DNA was found on plastic fragments from the handles of two knives used to kill a man which implicated two brothers. At trial, the forensic scientist felt that primary transfer was the most probable explanation. In Garmson, DNA was recovered from a victim of rape and sexual assault. After submitting the intimate samples to low copy number analysis, all but one of them was found to contain DNA belonging to Mr Garmson. The forensic scientist gave evidence that there was a strong match probability with the defendant with respect to two samples.

The Court of Appeal, in dismissing both appeals, held that DNA analysis based on amounts of DNA over 200 picograms can safely be assumed to be reliable and, in the absence of new scientific evidence, a challenge to the validity of the low copy number process (or indeed any other accredited process) of analysing low template DNA should no longer be permitted at trial.

Rule book

Where a challenge is to be taken, the Court of Appeal stressed that it expects the provisions of part 33.3 of the Criminal Procedure Rules 2010 to be followed in sequential order. This outlines the procedures parties should adopt when dealing with expert evidence and enables the court to control the way that evidence is handled.

In the context of DNA evidence, it provides important safeguards where a difference of opinion on the reliability and interpretation of DNA profiling exists. Central to this updated rule includes the following process:

• The reporting scientist for the prosecution must provide full details of what the evidence is and their opinion on that evidence. If there is a range of opinion which differs from this opinion, that range of opinion must be stated.
• If the evidence can be contradicted this has to be undertaken by a scientist of a similar experience and understanding of that area of science. That individual scientist must prepare his or her own report, and that report must be disclosed to the other side and to the court as soon as possible.
• Further, a statement which sets out the issues that are agreed upon and disagreed upon can go before the jury, not as evidence of agreed facts but to assist the jury's understanding of the expert evidence.

Future challenges

If the amount of DNA is below 200 picograms, there is an accepted risk that the profiles produced may be unreliable, and, as such, cases which fall into this category remain a valid area for challenge. For samples above 200 picograms, it may be possible to challenge the interpretation of DNA results; for example, if the sample has degraded or there is a challenge to the integrity of the storage or handling procedure.

If low template DNA evidence is to bechallenged, the following key points should be borne in mind:

• Only an appropriately qualified independent expert should consider the grounds for disputing the findings of the scientific evidence being brought either against the methodology, results or interpretation. This applies for experts appointed to perform any re-run of the test.
• Does the evidence in dispute meet the criteria for reliability set out in Reed and Garmson?
• The type of DNA testing used should be scrutinised so that an informed decision can be taken as to the applications of Reed and Garmson.
• The requirements of rule 33 of the Criminal Procedure Rules must be followed. Failure may result in the scientist or the expert who has not cooperated in the preparation of that statement be precluded from giving his or her own opinion evidence at all.

It is essential to remember that it is not just about the quantity of DNA involved but also the quality. Continuity and interpretation are the starting points for a practitioner to take into account when considering DNA evidence.

Practitioners should also read the additional cases which provide insight into how the courts deal with new developments in scientific evidence '“ see in particular: R v Doheny [1997] 1 Cr App R 369, R v Atkins [2009] EWCA Crim 1876 and R v Weller [2010] EWCA Crim 1085.

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