Experts wary of ‘DNA’ samples

FORENSIC evidence taken by Portuguese police from scientists in Britain appears to be behind the recent dramatic twists in the Madeleine McCann investigation.

But forensic experts warned yesterday that any evidence needed to be treated with caution until the full facts of the case were known.

The samples of DNA now at the centre of the investigation — and the subject of questions put to the McCanns — were taken from two sources, according to reports.

One is thought to be the Algarve apartment from where the four-year-old vanished, and the other is believed to be a hire car used by the McCanns 25 days after her disappearance.

If DNA with a close match to Madeleine was found in both locations, it would have raised serious questions.

But Alan Baker, a forensic scientist who has given expert evidence on the subject in court cases, said several important factors needed to be considered.

Mr Baker said the type of sample — whether actual blood or just a smear — was vital in interpreting any potential match.

“If they have found a hair follicle or a trace of blood at the scene then the implications could be immense but if it is only a smear then there are all sorts of issues involved.

“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,” he said.

Mr Baker added: “If it was not blood or a hair follicle or saliva then it makes it extremely difficult, as then you are just dealing with a trace or smear.

He said the science of matching DNA profiles was also made very difficult by the fact that members of the same family were involved.

If the match was less than perfect, as some newspaper reports have suggested, then it becomes more likely that any DNA is not Madeleine’s, but her siblings or parents.

“In most crime scenes, the individuals in question are not related so the DNA is completely different,” said Mr Baker.

“But in this case you have got members of the McCann family involved and they would have shared DNA with their offspring.”

He also pointed out that any cross-contamination of DNA had to be taken on board as a possible explanation if it was Madeleine’s profile in the car or flat.

For instance, anything Madeleine touched in the days before she went missing — perhaps her toys — might then have helped transfer her DNA, he said.

Mr Baker also urged caution over how the samples were actually obtained.

He said he was confident that any DNA testing by Britain’s Forensic Science Service (FSS) would have been reliable, but added they had no control over how any samples were collected: “If the crime scene was in Britain then I would be 100% confident in the integrity of those samples but if they were taken by poorly- trained people then there are all sorts of issues.”

He added: “Even though DNA is incredibly powerful it does not offer an answer to everything and sometimes it makes things more complicated because it adds more potential scenarios to the case.

“DNA is an amazing tool but it is the interpretation of that work which is the key thing.”

Mr Baker, of forensic science consultancy firm Bericon, said the degree of any potential match was also crucial to the investigation.

He said it could be that scientists used a specialised technique known as low copy number (LCN) tests to “amplify” the DNA found in the samples.

This sometimes only produces a partial profile that would leave a greater degree of statistical doubt over any match.

Another expert, Allan Scott, a lecturer at the University of Central Lancashire’s School of Forensic and Investigative Sciences, also warned that cross-contamination needed to be considered: “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.”