Tuesday, September 11, 2007


All summer long we have suffered with the parents of little Madeline McCann, missing in Portugal. Now, they are both suspects in the investigation. What is going on?

What is going on is that the Portuguese police seem to be going back to first principles. All police forces know as a matter of faith that the person most likely to do you harm is a member of your own family. That's just a fact. Not surprisingly they seem to have come late to this realisation. Because the McCanns are an ideal media couple - very respectable doctors, devoutly religious and photogenic, the idea that they might be responsible for Maddie's disappearance is hard to believe. But, as they say, truth is often stranger than fiction.

Fact is: there has been no concrete information about Maddie's disappearance since May. For all intents and purposes she may have been abducted by aliens. She just disappeared. That's one of the few indisputable, concrete facts we can cling to.

What seems to be swinging the investigation towards the parents is DNA evidence. However improbable, it is reported that the police have found Maddie's DNA in a car hired by the McCanns long after Maddie's disappearance. How can this be?

All sorts of theories have surfaced. The DNA came from Maddie's stuffed toy: items of her clothing were transported in the car: the parents used the car to transport Maddie's body somewhere and bury it. What is true is the case raises questions about DNA evidence that have been troubling me for some time.

Ever see the set of adverts on TV which show people acting like apes? The adverts go on to say that human DNA differs from chip DNA by considerably less than one percent. I wonder how many people actually know what this means in practical terms?

I wonder how many people know that the human genome shows almost no genetic variation. We are all remarkably closely related. At the same time, of course, we all have unique DNA sequences which set us apart from other humans. Except for identical twins, we all have a uniquely special set of DNA inherited from our parents. That's the theory anyway, and it fits the known scientific evidence quite well. But, is it foolproof?

Where the sample DNA consists of only minute quantities, or material that has suffered damage, the picture is not so clear. Nevertheless, we take the shibboleth that DNA evidence is the be all and end all of conclusiveness. If the DNA says you did it – you did it.

I've always thought this is very dangerous. Maddie's case highlights some real concerns. If there is DNA in the hire car and it is from Maddie, how did it get there? It's dangerous in the extreme to suppose it must have been deposited by the parents moving the body. Without other corroborating evidence, it should not be possible to convict anyone of a crime solely DNA evidence. If “your” DNA is found on a murder weapon and you know it wasn't you you will soon become a convert to the Scepticism Club. Probably a founder member!

The jury is out on Maddie's disappearance. It is possible the parents are involved/responsible. DNA evidence may help to clear this up. It cannot be used on its own to convict them – or, indeed, anyone else.

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