R v Deen

JurisdictionEngland & Wales
Judgment Date21 December 1994
Date21 December 1994
CourtCourt of Appeal (Criminal Division)

Court of Appeal

Before Lord Taylor of Gosforth, Lord Chief Justice, Mr Justice Owen and Mr Justice Latham


Criminal evidence - DNA test - "prosecutor's fallacy" explained

`Prosecutor's fallacy' in DNA test

The "prosecutor's fallacy" of confusing what was known as the DNA match probability with what was known as the likelihood ratio was explained by the Court of Appeal when explaining in outline the basic technicalities of the complex process of DNA (deoxyribonucleicacid) profile matching.

The court was giving reserved reasons for having allowed an appeal, quashed a conviction and ordered the retrial of a man convicted of three rapes who had been sentenced to 16 years imprisonment.

Mr Michael Mansfield, QC and Mr Robert Platts, assigned by the Registrar of Criminal Appeals, for the appellant; Mr Michael Shorrock, QC and Mr Howard Bentham for the Crown.

THE LORD CHIEF JUSTICE, giving the reasons of the court, said that the appeal against conviction was brought by leave of the single judge. The main evidence said to link the appellant with one of the rapes was DNA profiling which was crucial to the whole case. Failure on that count would inevitably mean failure on all counts.

The process of DNA profiling was extremely complex. Human cells in blood and in semen contained DNA. The process of DNA profiling started with DNA being extracted from the crime stain and also from a sample taken from the suspect.

In each case the DNA was cut into smaller lengths by specific enzymes. The fragments produced, which were radioactive, were sorted according to size by a process of electrophoresis. That involved placing the fragments in a gel and drawing them electro-magnetically along a track through the gel.

The fragments with smaller molecular weight travelled further than the heavier ones. The pattern thus created was transferred from the gel on to a membrane. Radioactive DNA probes, taken from elsewhere, which bound with the sequences of most interest in the sample DNA probe, were then applied.

After the excess of the DNA probe was washed off, an X-ray film was placed over the membrane to record the band pattern. That produced an auto-radiograph which could be photographed.

When the crime stain and sample DNA from the suspect were run in separate tracks through the gel, the resultant auto-radiographs could be compared. The two DNA profiles could then be said either to match or not.

Even if a number of bands corresponded exactly, any discrepancy between...

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3 cases
  • R v Reed; R v Garmson
    • United Kingdom
    • Court of Appeal (Criminal Division)
    • 21 December 2009
    ...it has been employed as a routine technique in the United Kingdom since 1987. The process as first used was described by Lord Taylor CJ in R v Deen (21 December 1993). In R v Doheny [1997] 1 Cr App Rep 369, Phillips LJ (as he then was) gave some general guidance about its interpretation, th......
  • R v SALLY Clark
    • United Kingdom
    • Court of Appeal (Criminal Division)
    • 11 April 2003
    ...statistics in a way that gave rise to the "prosecutor's fallacy" identified in relation to DNA statistical evidence in R v Deen, The Times 10 January 1994. 106 As to the first point, the Court of Appeal (at paragraph 155) concluded: "The existence of arguments against squaring was known to ......
  • The Queen Complainant v Alexander Bonnett Defendant [ECSC]
    • St Lucia
    • High Court (Saint Lucia)
    • 27 July 2010
    ...by the suspect. This process is complex…" 7 The process of DNA testing was succinctly described by Lord Taylor, CJ in Deen, The Times, January 10, 1994, and this description was adopted by Phillips, L.J. in Doheny and Adams, supra. I can do no better than to do the same. The learned Chief J......
1 books & journal articles
  • Presenting Probabilities in Court: The DNA Experience
    • United Kingdom
    • Sage International Journal of Evidence & Proof, The No. 1-4, December 1997
    • 1 December 1997
    ...the Court has recently created a ‘DNA exception’ to its ‘no probabilities’ rule: State v Bloom 516 NW 2d 159 (1994). R v Deen The Times, 10 January 1994; R v Dalby (unreported, CA no. 94/2819/W2 (1995)); R v Adam 119963 2 Cr App R 467; R v Doheny. R v Adums The Times, 14 August 19%. Though ......

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