McCann: forensics warning
Updated on 10 September 2007
Experts are urging caution be used over forensic evidence, that may lie at the centre of the Madeleine McCann investigation.
The recent charging of Kate and Gerry McCann in Portugal, naming them as 'arguidos', is reportedly based upon two samples of forensic evidence received by the Portuguese police.
However British forensic experts are urging that caution be used, until the full facts of the case are known.
There are two samples of DNA that have come under spotlight recently, one is believed to be from the apartment in the Algarve where Madeleine disappeared; and the second focuses on a hire care rented by the McCanns 25 days after her disappearance.
'When you get trace DNA it is incredibly difficult to interpret and that is the key point.'Alan Baker, forensic scientist
However it is the nature of the DNA that is raising doubts. If both samples were a close match, than the implications upon the case would be "immense".
Yet it is possible the DNA found in the car was just a smear or "trace" sample, which could have come a wider range of sources - including a McCann family member's own blood.
Alan Baker, a forensic scientist who provides expert evidence in court cases, warns:
"If they have just found traces of DNA in the vehicle or the flat then that offers up all sorts of explanations, and you have to look at how it could have got there.
"When you get trace DNA it is incredibly difficult to interpret and that is the key point."
Several newspaper reports have also suggested the DNA sample is less than perfect, which makes the matching of DNA profiles more difficult, as members of the same family are involved.
"The interesting thing about this is that in most crime scenes, the individuals in question are not related so the DNA is completely different," Mr Baker said.
"But in this case you have got members of the McCann family involved and they would have shared DNA with their offspring."
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There is also the question of cross-contaminiaton to consider when attempting to determine how a sample of DNA arrived at the scene.
It is possible that anything Madeleine touched days before she went missing, such as toys, may then have transferred her DNA to another point.
Another expert, Allan Scott, a lecturer at the University of Central Lancashire's School of Forensic and Investigative Sciences, also warned about the dangers of cross-contaminaion of DNA from sources outside the McCann family, such as investigating policemen who may have accidentally transferred something:
"DNA is so sensitive that if you and I met in the street and shook hands and then I went and committed some crime, then I could possibly leave your DNA at the scene," he said.