questfortrue
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UKGuy, although you were framing your question for AK, I wanted to add some detail from what I've studied.
As we already know, both samples of the DNA tested were mixed with JonBenét’s. In the first sample tested in the mid 90’s by Cellmark, they were unable to tell with certainty if the contribution of foreign DNA was from only one or more than one person. They ended up saying that if it were from a single person, then the Rs were excluded. Because it was an inconclusive piece of evidence, one doesn’t hear much about it now.
It seems the second sample is attributed to a single male. But the second sample was also degraded (degradation meaning the process of DNA breaking down into smaller and smaller fragments.) As I’ve learned, the relevant fact in regards to the second Distal Stain 007-2 DNA is that in addition to it being from a mixed sample it also featured allele dropout. (That fact suggests legal concerns, which I won’t go into here.)
Apologies if this is a repeat, but the second sample was at the lower range of what is usually preferred in analysis – 500 pg (picograms) of DNA material, plus there was the possibility of masking of some of the unknown and degraded contributor’s alleles behind JonBenét’s rich sample. All this has been discussed at one time or another, and I know I’m not really adding anything new. From what I’ve understood as a non-expert, the reason it’s relevant to mention masking is that it brings special issues to bear in difficult sample scenarios, i.e., mistakes can and do happen.
Something else I read recently might be worth mentioning. It pertains to evidence of this type being virtually flaunted in cases. It comes from a recent European consortium of researchers in forensic genetics and its application in case work and was presented in 2013.
- Transfer of DNA is much ‘easier’ than previously believed
- Often these will be full profiles
- Therefore, much more caution is needed in reporting
- E.g. the association of an activity such as stabbing, with a DNA profile can never
be definitively inferred simply by the presence of a DNA profile on a knife handle
-The relevance of the evidence and the probative value of the DNA profile are two
separate issues to be dealt with.
-Unexpected ease of spread of DNA profiles means that scientists should be very
cautious in reporting – it is suggested that evidence should not be inadvertently
weighted to suggest that an activity is associated with a profile in the absence of other corroborating evidence**
-Collection of background controls makes a lot of sense (but currently it is unlikely that this procedure is ever followed in practice).
**It’s what we’ve all been saying, but not everyone listens.
As we already know, both samples of the DNA tested were mixed with JonBenét’s. In the first sample tested in the mid 90’s by Cellmark, they were unable to tell with certainty if the contribution of foreign DNA was from only one or more than one person. They ended up saying that if it were from a single person, then the Rs were excluded. Because it was an inconclusive piece of evidence, one doesn’t hear much about it now.
It seems the second sample is attributed to a single male. But the second sample was also degraded (degradation meaning the process of DNA breaking down into smaller and smaller fragments.) As I’ve learned, the relevant fact in regards to the second Distal Stain 007-2 DNA is that in addition to it being from a mixed sample it also featured allele dropout. (That fact suggests legal concerns, which I won’t go into here.)
Apologies if this is a repeat, but the second sample was at the lower range of what is usually preferred in analysis – 500 pg (picograms) of DNA material, plus there was the possibility of masking of some of the unknown and degraded contributor’s alleles behind JonBenét’s rich sample. All this has been discussed at one time or another, and I know I’m not really adding anything new. From what I’ve understood as a non-expert, the reason it’s relevant to mention masking is that it brings special issues to bear in difficult sample scenarios, i.e., mistakes can and do happen.
Something else I read recently might be worth mentioning. It pertains to evidence of this type being virtually flaunted in cases. It comes from a recent European consortium of researchers in forensic genetics and its application in case work and was presented in 2013.
- Transfer of DNA is much ‘easier’ than previously believed
- Often these will be full profiles
- Therefore, much more caution is needed in reporting
- E.g. the association of an activity such as stabbing, with a DNA profile can never
be definitively inferred simply by the presence of a DNA profile on a knife handle
-The relevance of the evidence and the probative value of the DNA profile are two
separate issues to be dealt with.
-Unexpected ease of spread of DNA profiles means that scientists should be very
cautious in reporting – it is suggested that evidence should not be inadvertently
weighted to suggest that an activity is associated with a profile in the absence of other corroborating evidence**
-Collection of background controls makes a lot of sense (but currently it is unlikely that this procedure is ever followed in practice).
**It’s what we’ve all been saying, but not everyone listens.