[quote="jester"]Does it mean:-
1. The DNA matches 88% of Madeleine's therefore the DNA is not Madeleines.
2. The 88% of the DNA matches Madeleines and 12% of the DNA is too damaged to tell. Therefore is could be Madeleine's.
If the DNA could be Madeleine's with this 88% figure what exactly does that mean?
What is the likely hood of someone else having that same DNA match?
And what is the likelyhood of their DNA being in the car/appartment?
Does anyone know?[/quote]
************************************************************************************
Let us go through your questions one at the time. First, some preliminary information:
When DNA is extracted from a sample of body fluids, bone, tissue, etc. believed to belong to a missing person, forensic technicians speak of a certain number of "markers." The short definition of "marker" is a unique sequence of DNA having a known location. Different jurisdictions test different numbers of markers in order to determine whether there is a match between questioned DNA and a known sample of DNA from the person involved. In the UK the standard is 10. In other words, if a 10/10 match is found, it is considered that there is well over a 99% chance that the questioned DNA belongs to the person whose DNA is being tested.
I am not familiar with Portuguese law, but apparently they test for more than 10 markers, because press reports have repeatedly referred to 15 of 20 or 19 of 25 markers (or some other combination of numbers) that matched Madeleine's. Now, to address your questions:
1. The DNA matches 88% of Madeleine's therefore the DNA is not Madeleines.
This is not correct.
If there is a no-match on even ONE marker, the DNA could not be Madeleine's. If they had a 12% no-match, they would just say that it is not Madeleine's DNA, because it couldn't be.
2. The 88% of the DNA matches Madeleines and 12% of the DNA is too damaged to tell. Therefore is could be Madeleine's.
According to what we have read in the press, and assuming that the people who wrote the articles understand DNA analysis and were right about the numbers, this is the correct statement.
Sometimes DNA is so degraded (by sunlight, heat, the passage of time, fire, use of cleaning agents in an attempt to get rid of it, etc.) that it is not possible to extract the full number of markers normally used in a test. In that case, they test the number of markers they can extract. The results in those cases are expressed as a percentage. If 20 is the number of markers normally tested for and 15 match but 5 are too degraded to be usable, the match is said to be 15/20 or 75%. If 25 markers are normally tested for, 22 match and 3 were too degraded to use, the match is said to be 22/25 or 88%.
I have never worked a case where the DNA analysis was done by the FSS. In reports from labs that I worked with, DNA results were not expressed as negative, inconclusive, or positive, but were always given as either a no match, a percentage, or conclusive. (If one thinks about it, that would seem to be the more useful way to describe it, as lumping DNA that is a 50% match with DNA that is a 96% match under the category "inconclusive" does not really tell the tale.)
DNA results that show less than a perfect match still have probative value. The value would be somewhat subjective and would have to be viewed in the context of other evidence, but it is not accurate to say that it's all-or-nothing with a partial match on DNA when some of the markers were too degraded to test.
As to the likelihood of someone else having DNA that would match Madeleine's DNA on 88% of the markers tested, I will refer to information from the FBI. The FBI maintains a DNA database called CODIS which stores DNA collected from convicted offenders for testing against questioned DNA samples obtained from crime scenes. CODIS tests for 13 markers. The FBI web site says, ""The probability of two different people matching at all 13 CODIS sites is virtually zero."
There have been some cases where there have been matches on some markers. This is especially true when siblings are involved, as their DNA will be more alike than that of two individuals compared at random; we must keep in mind that, even though the business about Sean and Amelie's diapers being a possible source of DNA seems far-fetched, they were in the hire car. However, the Council for Responsible Genetics reported in early 2007 that there had never been a known case, even among siblings, where there was a 13/13 match to the markers stored in the CODIS database. Since we appear to be talking about more than 13 markers in this case, a 15/15 match or a 19/19 match would be even more unlikely.
My personal opinion is that, if press reports are correct and they have an 88% match between Madeleine's DNA and the DNA found in the hire car, this is good evidence. Not a smoking gun, not a slam-dunk, but definitely a piece of the puzzle. |
Free Forum Hosting
Important Announcement
First 