A powerful tool - but not infallible
DNA profiling is the most powerful forensic tool available to police and scientists today. Invented by Sir Alec Jeffreys, the Leicester University geneticist, the technique was first used to solve the murders of the Leicestershire schoolgirls Dawn Ashworth and Lynda Mann in 1988. In those days, fairly substantial amounts of biological material were needed to create DNA profiles. Since then scientists have perfected the technology so that they can create a profile from a few cells, invisible to the naked eye, that have been left behind at a crime scene. Holding a glass will leave enough DNA on its surface to create a profile, for example.
The technology exploits the fact that small sections of our DNA repeat themselves over and over and that different people have different numbers of repeats. These genetic 'stutters' can be counted, producing a digital readout.
Current tests - known as 'LCN' (low copy number) tests - produce a sequence of 20 numbers. Such a readout is highly specific to an individual. The chances of a stranger's DNA exactly matching each of the 20 numbers that make up your profile are about one billion to one.
Equally importantly, the ability to turn a person's DNA into a readout means that the numbers that pinpoint a person from their genes can be stored in computers. Britain's currently has the DNA records of more than 4 million people and matches between individuals and crime-scene samples are now frequently used to channel investigations in particular directions.
But using DNA profiles on their own to try to establish guilt can be a fraught business. For a start, the technology's ability to create profiles from microscopic scraps of biological material means tests are highly vulnerable to contamination. Forensic scientists have to take extraordinary precautions to counter the problem and therefore work in sterile laboratories; the sight of the 'experts' on TV's CSI: Crime Scene Investigation eating in the lab or letting their hair fall over samples turns most British forensic scientists apoplectic.
This point is of key importance in the case of Madeleine McCann. Scientists may have found her DNA profile in the car that her parents hired 25 days after she went missing, but that tells them nothing about the source of that DNA. A few skin cells from one of Madeleine's toys - such as her 'cuddle cat', which has been carried around by Kate McCann - could have fallen into the vehicle, for example.
In addition, it is now known that the DNA found in the McCanns' hire car, and also in the family's flat, generated a readout for only 15 numbers out of a possible 20, although these all matched corresponding numbers on Madeleine's DNA.
Nevertheless, this failure to produce a complete readout is intriguing and suggests the biological traces found by scientists are tiny and degraded. The finding also suggests an outside possibility that the DNA could have come from the McCanns' other children, though this idea is dismissed by detectives.
The point about DNA profiling is that it is a potent technique for highlighting promising approaches for detectives to follow. But other evidence is often needed to corroborate what the DNA evidence is indicating.
The technology exploits the fact that small sections of our DNA repeat themselves over and over and that different people have different numbers of repeats. These genetic 'stutters' can be counted, producing a digital readout.
Current tests - known as 'LCN' (low copy number) tests - produce a sequence of 20 numbers. Such a readout is highly specific to an individual. The chances of a stranger's DNA exactly matching each of the 20 numbers that make up your profile are about one billion to one.
Equally importantly, the ability to turn a person's DNA into a readout means that the numbers that pinpoint a person from their genes can be stored in computers. Britain's currently has the DNA records of more than 4 million people and matches between individuals and crime-scene samples are now frequently used to channel investigations in particular directions.
But using DNA profiles on their own to try to establish guilt can be a fraught business. For a start, the technology's ability to create profiles from microscopic scraps of biological material means tests are highly vulnerable to contamination. Forensic scientists have to take extraordinary precautions to counter the problem and therefore work in sterile laboratories; the sight of the 'experts' on TV's CSI: Crime Scene Investigation eating in the lab or letting their hair fall over samples turns most British forensic scientists apoplectic.
This point is of key importance in the case of Madeleine McCann. Scientists may have found her DNA profile in the car that her parents hired 25 days after she went missing, but that tells them nothing about the source of that DNA. A few skin cells from one of Madeleine's toys - such as her 'cuddle cat', which has been carried around by Kate McCann - could have fallen into the vehicle, for example.
In addition, it is now known that the DNA found in the McCanns' hire car, and also in the family's flat, generated a readout for only 15 numbers out of a possible 20, although these all matched corresponding numbers on Madeleine's DNA.
Nevertheless, this failure to produce a complete readout is intriguing and suggests the biological traces found by scientists are tiny and degraded. The finding also suggests an outside possibility that the DNA could have come from the McCanns' other children, though this idea is dismissed by detectives.
The point about DNA profiling is that it is a potent technique for highlighting promising approaches for detectives to follow. But other evidence is often needed to corroborate what the DNA evidence is indicating.
