Tuesday, 5 August 2008

The McCann Case: Why 15 over 19 DNA components isn't enough

As it is front page news at almost every British newspaper, it's interesting to know with more detail what the scientists from Forensic Science Service told Portuguese police, on September 4, 2007, a few days before Gerry and Kate McCann were named "arguidos."

According to a document from FSS, signed by Mr. J.R. Lowe, analysis to the samples collected at the boot of the Renault Scenic produced a complex Low Copy Number DNA result which appeared to have originated from at least three people. Madeleine's DNA profile had 19 elements, each one represented by a peak on a chart, explains Mr. Lowe.

As Madeleine has inherited a same DNA component from both parents, the number of peaks in the chart is 19, instead of 20. Those 2 components appear, in the chart, as only one peak.

The FSS document states that of those 19 components, 15 are present within the result of the analysis to that specific sample. But the total number of components is 37, because there are at least three contributors, may be up to five, in the sample.

Mr. J.R. Lowe's opinion was that the result is too complex to have a meaningful interpretation or conclusion.

But, concerning the question asked, when the samples were sent to FSS – if it could be or not Madeleine's DNA – the FSS scientist admits that it would be very simple to say yes, because of the number of components, within the sample, that are also present in Madeleine DNA profile – 15 over 19.

However, as Mr. J.R. Lowe emphasizes, in the mentioned document, the scientists need to consider if the match is genuine and legitimate. Meaning, to be sure if either Madeleine's DNA was deposited in the car or if the result matched Madeleine's DNA just by chance.

To explain in more detail this question, the FSS scientist reminds that individual components of Madeleine's DNA are also present within the profile of many of the scientists that work in the Birmingham laboratory. Mr. Lowe even refers to his own DNA profile, as an evidence of that.

As the sample analysed has a mixture of elements from more than two persons, it is not possible, according to the FSS expert, to determine or evaluate which specific components pair with each other. Another difficulty mentioned is the fact that it's not possible to separate the components out into three individual DNA profiles.

As a conclusion, Mr. J.R. Lowe wrote that he could not answer the question if the partial match was genuine or just a chance match.

Duarte Levy and Paulo Reis

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