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What does a DNA match tell us?

By Channel 4 News

Updated on 11 September 2007

Portuguese police have found DNA matching missing Madeleine in the McCann's hire car, reports say. Is this a key breakthrough in the case?

Q. What has been found in the car?

A. Over the weekend Portuguese police sources said that had discovered DNA samples that were an 80 per cent match to missing Madeleine McCann.

Then, late yesterday reports claimed the police had found samples that were a 99 per cent match. The hire car, a Renault Scenic, was used by Kate and Gerry McCann 25 days after the girl went missing.

Q. So what does the 80 per cent match tell us?

A. Not a lot.

An 80 per cent match indicates that 16 of 20 markers used in analysis are the same. But not only does that suggest the sample was small, but that this type of 'match' doesn't rule out other family members - notably Madeleine's siblings Sean and Amelie.

Q. And the 99 per cent match?

A 99 per cent match could be significant - if a large sample has been discovered.

If that's the case then it is unlikely to have been transferred by another source. If it's a small sample, however, it could easily have been transferred when the McCann's were moving some of Madeleine's belongings such as her 'cuddle cat'.

Equally, the sample could be a hair follicle stuck to clothing or a child seat.

Q. Can you tell if DNA comes from a dead body?

A. No. In the words of Allan Scott, a lecturer at the University of Central Lancashire's School of Forensic and Investigative Sciences: "It is about your blood group, which does not change because you have died. A person's DNA profile does not change just because they die.

He added: "You can tell if the DNA has degraded but that could happen fairly quickly depending on the environment, if for example it is getting baked in the sun.

"It may be possible to see that a DNA sample has degraded because of where it was or the length of time between it being deposited and recovered but that does not tell you if the person is dead or alive."

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