Joined: 10 Sep 2007
Posts: 984
|  Posted: Fri Nov 02, 2007 5:03 pm Post subject: DNA II - Are These Results Real? |   |
| I am receiving questions about the report in the Daily Mail regarding the DNA results. Rather than trying to answer them one by one and possibly dealing with the same question more than once, I thought it better to start a new thread. Anyone interested in the latest DNA reports who has not read the previous thread entitled "DNA - What It Will Tell Us and What It Won't" might consider bringing that up on search and reading it for reference purposes. I decided not to add these questions to that thread as it is already so long and would require considerable scrolling to get to the heart of the matter, i.e., questions relating to the latest report.
Questions:
(1) Do you believe this latest report in the Daily Mail about the DNA is true?
I've no idea whether it is true or not, and I think we should wait for better information before drawing any conclusions. I did not see anything in the Daily Mail article that seemed patently impossible, but there've been so many of these reports now that I feel it's prudent to reserve judgment. This could be nothing more than another "source" looking for his 15 minutes.
(2) The report says there was too much DNA in the hire car for it to have been transferred accidentally. How is it possible to determine that?
The actual quotation in the article is, "Sources also indicate that the volume of material found is too great for it to have been transferred via Madeleine's toys or clothing or from her parents' possessions."
Whilst secondary transfer of DNA is possible, there are situations where common sense would seem to rule out such a scenario. For example, if a towel containing a large amount of dried blood were laid in the boot of a car, one would expect a greater volume of DNA to be transferred to the car's carpeting than one would find if, say, the blood came from a small amount that had dropped onto a parent's clothes whilst he/she was trying to assist a child with a nosebleed. This would be a somewhat subjective decision, but there comes a point where it is no longer logical to assume that the volume of DNA found could have fallen off a stuffed animal or out of the band of a hat simply because there is so much of it.
(3) I am sure I recall your saying before that DNA tests will not indicate whether the DNA came from a person living or dead. Yet the article seems to indicate that is precisely what these tests show. Can you explain that?
(Sighs deeply.) The information provided in press reports, including the latest, is not really sufficient to explain it, but I'll have a go. Precisely what the article says is this: "Tests on the bodily fluids found in the hire car are understood to show that they came from a corpse and that the body was moved."
DNA is not like cancer or bacterial cells that one can actually observe growing in a petri dish. All DNA is in a sense "dead," in that it is no longer part of a living cell. If one were to take a cheek swabbing from a hospital patient an hour before he died, another just after death, and sequence the DNA from both, they would look identical.
It is not in looking at the actual DNA itself but in working with the material from which one is attempting to extract it that one can usually tell where it came from. DNA can be extracted from many types of bodily fluids - blood, semen, saliva, fluids that lubricate mucous membranes, etc. - even in some cases from sweat and tears. There have been several previous reports that the DNA found in the boot of the McCanns' hire car came from "corpse fluid" or "decomposition fluids." These are not actual medical terms so far as I am familiar, and it appears that in the interest of remaining civilised, newspaper editors are using them to avoid being too descriptive about the second stage of decomposition, in which tissues begin to liquify, fluid from the lungs comes from the nose and mouth, etc. In the interests of simple decency I am not going into too great detail here, but suffice it is to say that if one were given a sample of dried fluids of this type and told to attempt to extract DNA from them, one would probably know what one was dealing with.
"The body was moved" part I think likely to come not from analysis of DNA itself but from the fact that some substance, probably dirt, was found mixed with the material from which the DNA was extracted.
(4) What does it mean when it says that the tests have produced no evidence to indicate involvement of another person?
It means that no other person's DNA was found in the material from which Madeleine's DNA was obtained. It does not mean that no other person could have been involved in Madeleine's death. To provide a hypothetical example: If someone smothered Madeleine with a pillow whilst wearing gloves and her DNA were obtained from the "decomposition fluids" mentioned above, it is unlikely that her killer's DNA would be mixed with hers.
(5) What does it mean when it refers to a "second batch of testing"?
I've two different ideas about this. One is that some results were already completed and possibly forwarded previously to Portugal, where they are being held pending arrival of the remainder of the test results (the "second batch").
The other is that the FSS may be running the DNA analysis multiple times. They are almost surely using low copy number STR analysis in this case, and one can be more confident of the results if the test is run more than once - as a matter of fact, they are almost always repeated and sometimes run three times. This is a laborious, time-consuming, but ultimately quite accurate process (see the thread "DNA - What It Will Tell Us and What It Won't" for a bit of an explanation about how it works.)
(6) The DM report says that the material is being held in the UK in case there are requests for further work to be carried out. What might this be?
I'm guessing here. Perhaps the PJ might ask for the LCN/STR analysis referenced in #5 above to be done one more time to establish a greater degree of accuracy in this extremely high-profile case. Also, if Madeleine's body is eventually found, enough time has elapsed that it would probably be necessary to identify her by DNA analysis; this would presumably also be done by the FSS.
(7) You have always urged caution in interpreting DNA results, but if this report is true, it is damning evidence against the McCanns, is it not?
One must always be cautious in interpreting DNA results, not because they aren't accurate, but because they're just one part of the picture. A perception has arisen in society that DNA is the end-all and be-all of forensics. Well, it's quite good and better than much of what was used in the past, but again, it's just one piece of a puzzle.
What I would say is this: If this report is correct and DNA established to be Madeleine's was indeed found in the hire car, in such amounts that it is not reasonable to conclude that it came from secondary transfer, then the McCanns have a hard job of explaining to do. I'll leave it at that.
We shall see. | |
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