R v Deen
| Jurisdiction | England & Wales |
| Judgment Date | 21 December 1994 |
| Date | 21 December 1994 |
| Court | Court 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
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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