R v Doheny

JurisdictionEngland & Wales
Judgment Date31 July 1996
Judgment citation (vLex)[1996] EWCA Crim J0731-1
CourtCourt of Appeal (Criminal Division)
Docket NumberNO: 95/5297/Y2
Date31 July 1996
Alan James Doheny
Gary Adams

[1996] EWCA Crim J0731-1


Lord Justice Phillips

Mr Justice Jowitt and

Mr Justice Keene

NO: 95/5297/Y2



Royal Courts of Justice

The Strand

London WC2

MR WEBSTER QC appeared on behalf of the Appellant Doheny

MR G COOKE appeared on behalf of the Appellant Adams

MR M SHORROCK QC and MS R BRAND appeared on behalf of the Crown.


Wednesday, 31st July 1996


This is the judgment of the Court, There are before the Court two appeals against conviction. In each case the Prosecution placed substantial reliance upon the results obtained by comparing DNA profiles obtained from a stain left at the scene of the crime with DNA profiles obtained from a sample of blood provided by the appellant. In each case leave to appeal was given long out of time to enable the appellant to advance arguments based on the possibility of shortcomings in the DNA evidence and the manner in which it was presented to the jury which were neither appreciated at the time of the trial nor when consideration was given to applying for leave to appeal against conviction after the trial. We have received fresh expert evidence on these matters adduced on behalf of both the Appellants and the Crown.


Before turning to the facts of the individual appeals we propose to make some general comments about DNA testing, the conclusions that can properly be drawn from such testing and the manner in which those conclusions should be presented to the jury.


DNA Testing


Deoxyribonucleic acid, or DNA, consists of long ribbon-like molecules, the chromosomes, 46 of which lie tightly coiled in nearly every cell of the body. These chromosomes —23 provided from the mother and 23 from the father at conception, form the genetic blueprint of the body. Different sections of DNA have different identifiable and discrete characteristics. When a criminal leaves a stain of blood or semen at the scene of the crime it may prove possible to extract from that crime stain sufficient sections of DNA to enable a comparison to be made with the same sections extracted from a sample of blood provided by the suspect. This process is complex and we could not hope to describe it more clearly or succinctly than did Lord Taylor CJ in the case of Deen (transcript: 21st December 1993), so we shall gratefully adopt his description.


"The process of DNA profiling starts with DNA being extracted from the crime stain and also from a sample taken from the suspect. In each case the DNA is cut into smaller lengths by specific enzymes. The fragments produced are sorted according to size by a process of electrophoresis. This involves placing the fragments in a gel and drawing them electromagnetically along a track through the gel. The fragments with smaller molecular weight travel further than the heavier ones. The pattern thus created is transferred from the gel onto a membrane. Radioactive DNA probes, taken from elsewhere, which bind with the sequences of most interest in the sample DNA are then applied. After the excess of the DNA probe is washed off, an X-ray film is placed over the membrane to record the band pattern. This produces an auto radiograph which can be photographed. When the crime stain DNA and the sample DNA from the suspect have been run in separate tracks through the gel, the resultant auto-radiographs can be compared. The two DNA profiles can then be said either to match or not."


Even if a number of bands correspond exactly, any discrepancy between the profiles, unless satisfactorily explained, will show a mis-match and will exclude the suspect from complicity. Thus the first stage in seeking to prove identity by DNA profiling is to achieve a match.


The characteristics of an individual band of DNA will not be unique. The fact that the identical characteristic of a single band are to be found in the crime stain and the sample from the suspect does not prove that both have originated from the same source. Other persons will also have that identical band as part of their genetic make-up. Empirical research enables the analyst to predict the statistical likelihood of an individual DNA band being found in the genetic make-up of persons of particular racial groups 'the random occurrence ratio'.


As one builds up a combination of bands, the random occurrence ratio becomes increasingly more remote, by geometric progression. Thus if two bands, each of which appear in 1 in 4 of the population are combined, the combination will appear in 1 in 16 of the population, and if to these are added a further band that is found in 1 in 4 of the population, the resultant combination will appear in 1 in 64 of the population. This process of multiplication is valid on the premise that each band is statistically independent from the others. The frequency ratio of the blood group is a factor which is statistically independent and thus this can also validly be used as a multiplier. If the DNA obtainable from the crime stain permits, it may be possible to demonstrate that there is a combination of bands common to the crime stain and the suspect which is very rare. For instance, it may be that the match achieved with the crime stain is one which has a statistical probability of existing in the case of only one in a million of the populace. We shall take a match probability, or random occurrence ratio, of one in a million as an example to demonstrate the conclusions that can properly be drawn from such data and those which cannot. We shall start with the latter.


"The Prosecutor's Fallacy"


It is easy, if one eschews rigorous analysis, to draw the following conclusion:


1) Only one person in a million will have a DNA profile


which matches that of the crime stain.


2) The Defendant has a DNA profile which matches the crime




3) Ergo there is a million to one probability that the


Defendant left the crime stain and is guilty of the




Such reasoning has been commended to juries in a number of cases by Prosecuting Counsel, by judges and sometimes by expert witnesses. It is fallacious and it has earned the title of "The Prosecutor's Fallacy". The propounding of the Prosecutor's Fallacy in the course of the summing up was the reason, or at least one of the reasons, why the appeal against conviction was allowed in Deen. The nature of that fallacy was elegantly exposed by Balding and Donnelly in 'The Prosecutor's Fallacy and DNA Evidence' [1994] CLR 711. It should not, however, be thought that we endorse the calculations on pages 715 and 716 of that article.


Taking our example, the Prosecutor's Fallacy can be simply demonstrated. If one person in a million has a DNA profile which matches that obtained from the crime stain, then the suspect will be one of perhaps 26 men in the United Kingdom who share that characteristic. If no fact is known about the Defendant, other than that he was in the United Kingdom at the time of the crime the DNA evidence tells us no more than that there is a statistical probability that he was the criminal of 1 in 26.


The significance of the DNA evidence will depend critically upon what else is known about the suspect. If he has a convincing alibi at the other end of England at the time of the crime, it will appear highly improbable that he can have been responsible for the crime, despite his matching DNA profile. If, however, he was near the scene of the crime when it was committed, or has been identified as a suspect because of other evidence which suggests that he may have been responsible for the crime, the DNA evidence becomes very significant. The possibility that two of the only 26 men in the United Kingdom with the matching DNA should have been in the vicinity of the crime will seem almost incredible and a comparatively slight nexus between the Defendant and the crime, independent of the DNA, is likely to suffice to present an overall picture to the jury that satisfies them of the Defendant's guilt.


The reality is that, provided there is no reason to doubt either the matching data or the statistical conclusion based upon it, the random occurrence ratio deduced from the DNA evidence, when combined with sufficient additional evidence to give it significance, is highly probative. As the art of analysis progresses, it is likely to become more so, and the stage may be reached where a match will be so comprehensive that it will be possible to construct a DNA profile that is unique and which proves the guilt of the Defendant without any other evidence. So far as we are aware that stage has not yet been reached.


The cogency of DNA evidence makes it particularly important that DNA testing is rigorously conducted so as to obviate the risk of error in the laboratory, that the method of DNA analysis and the basis of subsequent statistical calculation should —so far as possible —be transparent to the Defence and that the true import of the resultant conclusion is accurately and fairly explained to the jury.


The Role of the Expert


Mr Alistair Webster QC, on behalf of Doheny, has made the following suggestions as to the procedure which should be followed in relation to DNA evidence:


1. The scientist should adduce the evidence of the DNA comparisons together with his calculations of the random occurrence ratio.


2. Whenever such evidence is to be adduced, the Crown should serve upon the Defence details as to how the calculations have been carried out which are sufficient for the defence to scrutinise the basis of the calculations.


3. The Forensic Science Service ("FSS") should make available...

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