I have started having a wee look about, re DNA evidence. I am probably going to get out of my depth very quickly so if anyone has more expertise than me I would be glad if they would help me out. Complicated subject 
The first thing I found is this article
http://www.bioforensics.com/conference04/Transfer/index.html
Which suggests that transfer of DNA can happen quite easily. It is alarming because if this is the case then it is difficult to see how any DNA evidence can be relied upon to the extent we do in practice. However there is no date on this article. I noticed that the references go no later than 2001, so presumably it was written in that year or the next. At least I think that is a reasonable assumption.
I followed the links. The first one is a letter published in "Nature" dated June 1997. it reports the findings of some work done to investigate DNA transfer and on the basis of that speaks to the need for great caution in interpreting small samples. It is very short (being a letter) and I have not yet discovered whether was a subsequent paper.
The second link concerns techniques of obtaining evidence from a very small sample (single cell it says, which I find astonishing). It seems to be the first technique for handling such small samples but the results are positive even so. The authors identify some problems which meant that they only got acceptable profiles in 2/3 of the cells tested and they pointed to problems with amplification and allele "drop out" which meant the work needed validation before it could be used in forensics. It also emphasises the centrality of stringent precautions in collecting and processing such samples because of the danger of contamination.
This letter was also published in "Nature" in 1997. It is my understanding that 1997 is a very long time ago in this field and I think it is quite likely there has been progress since then. I have not had time to investigate more recent work but it seems to me that if they could do that with that degree of success in 1997, then the idea that a very small sample cannot yield reliable results now has to be false. This matters because of the different views about the knife (expressed by Dr Stefanoni and Dr Johnson), discussed upthread.
Of course it may be that subsequent work did not validate these findings: or that other problems have emerged: but in that case I would have expected an internationally respected person like Dr Stefanoni to be aware of that, and that she would not have testified as she did.
http://www.bioforensics.com/conference04/Transfer/FingerprintsFromSingleCells.pdf
The third link is a direct challenge to the first.
http://www.bioforensics.com/conference04/Transfer/SecondaryTransferStudy.pdf
Again this is quite a long time ago and it is a bit technical for me to follow it properly. The bit I gleaned which seems to be relevant to this discussion is in the conclusion, where it says: "the manufacturer [of the test used] recommends using a minimum of 250pg (35 cells)..." That was in 1999. Dr Johnson claimed that the material found on the knife amounted to " 20 human cells or fewer" and this sounds really dodgy to a lay person. But now I know that a) 10 years ago the manufacturers were recommending a minimum of 35 cells: and b] that there were promising results from analysis of single cell samples in 1997. Clearly 20 or fewer is about half of the manufacturers' recommendation in 1999 (though I do not know what Dr Johnson bases her statement about the amount on) This is the problem with soundbite evidence presented to the public. We have no background and no context against which to judge it.
But the article does not say that a smaller sample will give a less reliable result, so far as I can tell: it says it will give no result. Or at least that is how I interpret " at 125 pg or less, peak heights are close to background". I freely admit I may be reading that incorrectly. Dr Stefanoni did not get no results: she got strong results which she was confident in reporting. She (and at least one other expert) did not think there was doubt about the results; and I reiterate that she is internationally respected.
If anyone else has knowledge of the use of forensic DNA and the tests and limits of that I would really be glad if they would pipe up here: but I will do what I can to find more recent work that I can understand
http://www.bioforensics.com/conference04/Transfer/FingerprintsFromSingleCells.pdf
The first thing I found is this article
http://www.bioforensics.com/conference04/Transfer/index.html
Which suggests that transfer of DNA can happen quite easily. It is alarming because if this is the case then it is difficult to see how any DNA evidence can be relied upon to the extent we do in practice. However there is no date on this article. I noticed that the references go no later than 2001, so presumably it was written in that year or the next. At least I think that is a reasonable assumption.
I followed the links. The first one is a letter published in "Nature" dated June 1997. it reports the findings of some work done to investigate DNA transfer and on the basis of that speaks to the need for great caution in interpreting small samples. It is very short (being a letter) and I have not yet discovered whether was a subsequent paper.
The second link concerns techniques of obtaining evidence from a very small sample (single cell it says, which I find astonishing). It seems to be the first technique for handling such small samples but the results are positive even so. The authors identify some problems which meant that they only got acceptable profiles in 2/3 of the cells tested and they pointed to problems with amplification and allele "drop out" which meant the work needed validation before it could be used in forensics. It also emphasises the centrality of stringent precautions in collecting and processing such samples because of the danger of contamination.
This letter was also published in "Nature" in 1997. It is my understanding that 1997 is a very long time ago in this field and I think it is quite likely there has been progress since then. I have not had time to investigate more recent work but it seems to me that if they could do that with that degree of success in 1997, then the idea that a very small sample cannot yield reliable results now has to be false. This matters because of the different views about the knife (expressed by Dr Stefanoni and Dr Johnson), discussed upthread.
Of course it may be that subsequent work did not validate these findings: or that other problems have emerged: but in that case I would have expected an internationally respected person like Dr Stefanoni to be aware of that, and that she would not have testified as she did.
http://www.bioforensics.com/conference04/Transfer/FingerprintsFromSingleCells.pdf
The third link is a direct challenge to the first.
http://www.bioforensics.com/conference04/Transfer/SecondaryTransferStudy.pdf
Again this is quite a long time ago and it is a bit technical for me to follow it properly. The bit I gleaned which seems to be relevant to this discussion is in the conclusion, where it says: "the manufacturer [of the test used] recommends using a minimum of 250pg (35 cells)..." That was in 1999. Dr Johnson claimed that the material found on the knife amounted to " 20 human cells or fewer" and this sounds really dodgy to a lay person. But now I know that a) 10 years ago the manufacturers were recommending a minimum of 35 cells: and b] that there were promising results from analysis of single cell samples in 1997. Clearly 20 or fewer is about half of the manufacturers' recommendation in 1999 (though I do not know what Dr Johnson bases her statement about the amount on) This is the problem with soundbite evidence presented to the public. We have no background and no context against which to judge it.
But the article does not say that a smaller sample will give a less reliable result, so far as I can tell: it says it will give no result. Or at least that is how I interpret " at 125 pg or less, peak heights are close to background". I freely admit I may be reading that incorrectly. Dr Stefanoni did not get no results: she got strong results which she was confident in reporting. She (and at least one other expert) did not think there was doubt about the results; and I reiterate that she is internationally respected.
If anyone else has knowledge of the use of forensic DNA and the tests and limits of that I would really be glad if they would pipe up here: but I will do what I can to find more recent work that I can understand
http://www.bioforensics.com/conference04/Transfer/FingerprintsFromSingleCells.pdf
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