So, trying to solve a crime you can choose to test either who the material is from or what part of an unknown person it is from.
Seems like the obvious choice is to see whose DNA is on the blade.
"It was perhaps the only
sound decision she made, she decided to test to see who 36B belonged to.
That test said it was Meredith's."
Of course high resolution photographs of the blade reveal grooves any sighted person can see for themselves.
http://truejustice.org/ee/images/perugia/frontpage105/10556.pdf
There is a very real problem with not knowing what it is from. This is the crux of the concern with LCN results. Stephanoni was simply wrong in not splitting the sample to replicate the result. Because of drop out and drop in of alleles at low levels the profile may be artifactual. If she had been familiar with the methodology she would have split the sample to allow a second or even third run there was sufficient DNA to do this. Only if both runs give the same profile can it be accepted.
There are many articles describing the issues I quote from a couple.
"The use of DNA profiling has revolutionised the use of science in legal cases. DNA profiles produced from manufactured kits such as SGMPlus and Identifilier are routinely used as evidence in criminal cases. The kits are designed and validated by the manufacturer to operate within a specific range of amounts of DNA, typically 0.5 – 2.5ng (a nanogram is a 1,000th of a millionth of a gram). The kits work by copying (amplifying) the DNA molecules contained in a sample a number of times to produce enough to be detected in the analyser. The chemistry used by the kits is capable of amplifying just one molecule of DNA. By varying the conditions under which the kit is used some claim to be able to produce profiles from much lower amounts of starting (template) DNA. The general term for these techniques is variously referred to as Low Copy Number (LCN) or Low Template DNA (LTDNA) analysis, although some restrict the term LCN to the process used by the FSS Ltd which uses 34 cycles of amplification instead of the routine 28-cycle process recommended by the manufacturer. However, with very low numbers of template DNA molecules the process may fail to amplify the template. This leads to a number of problems in the interpretation of the resulting profiles. These are caused mostly by sampling, or stochastic, errors caused by the failure of the chemistry to work effectively with such low numbers leading to poor reproducibility of the results.
By way of analogy, an aeroplane, designed on the principles of flight, will fly and perform satisfactorily within the parameters of its design. As the speed lowers, there is no change to the aeroplane’s performance according to the laws of aerodynamics, but as the speed lowers further the aircraft will first suffer a loss of control and then simply stop flying (this is termed the stall speed). A graph of the flying performance of the aircraft will be continuous, but will show a precipitous fall-off at the stall speed. The stochastic threshold is the DNA profiling equivalent of this sudden change in performance. Profiling above the stochastic threshold produces consistent and reliable results. Below the stochastic threshold, reliability fails. It is disingenuous to equate the performance of ANY profiling technique above the stochastic threshold, to the performance below; that now appears to have been recognised scientifically and legally."
http://www.theforensicinstitute.com/news/low-copy-number-dna-and-the-forensic-institute.html
"As much as the DNA technologies created controversy and challenges when they were introduced, LCN DNA has produced its very own set of problems. Not least among these is the limited number of providers of this technology. In many cases they are working with old, degraded, or sub-microscopic volumes of material.
Many, if not all, of these old, or “cold”, cases occurred before DNA forced a rethink of the possibilities for contamination of evidence. Exhibits were collected with little regard for who was handling them or the possibilities of cross contamination from suspects to items via the investigating officer. Even the laboratory environment or procedures would not be designed to protect against the transfer of such low amounts of material. This was not negligence; it just didn’t consider the possibility of such traces becoming important.
In forensic science the fact to be established is that the DNA profile originated from the material recovered from a crime scene or a suspect, not the investigator, the laboratory, packaging, or analytical instruments. A “negative control” is set up by simply processing a “blank” sample that has no DNA. All being well, this control will not show any DNA. The presence of DNA in the negative control illustrates that there has been a source of contamination in the analytical method. It does not, of itself, show where that occurred, merely that it has. The tradition over many years has been, for very sound reasons, that anything found in the “negative control” invalidates the analysis.
There are now some who argue that this principle cannot be applied to LCN DNA analysis, because even in a tightly controlled analytical procedure a significant number of supposedly negative controls give a positive result, i.e. they indicate the presence of DNA.
The issue of course is that if it cannot be established that the DNA has been introduced during the analysis, how can any of the DNA found in the crime stains be shown NOT to have come from the procedure rather than the scene?
Lastly, the very small amounts of DNA and the vagaries of the method mean that it is frequently the case that replicate samples, that should produce the same results, don’t. The process gets around this difficulty by simply taking a vote of three replicates. DNA types found in two of the three are regarded as real and counted in the “consensus” profile. We use the consensus result as the basis of the statistical calculation of how rare a combination is in the population at large – in effect the probative value of the DNA evidence."
http://www.journalonline.co.uk/Magazine/52-2/1003857.aspx
Whatever is said on other sites DNA does float around there is adequate evidence published of secondary and tertiary transfer. In-laboratory contamination is a reality, the European standards for forensic science say...
"Environmental monitoring procedures should be written and records maintained. Monitoring can be done by swabbing instruments (centrifuges, vortex, ...), benches where exhibits are examined, door knobs and any other item relevant to the work done in the area where the monitoring is performed."
"Blank/negative controls will be used for every series of experiments.
Separate batches must be processed for reference and crime scene samples.
Intra and inter-batch contamination checks should be done."
http://www.enfsi.eu/sites/default/f...e_contamantion_prevention_final_-_v2010_0.pdf
The negative controls and environmental controls need to be negative, the court (and defence) need to know that they were done, and the results. It really is not adequate for Stephanoni to stand up in court and say there has been no contamination. How does she know? When did she last check the records. Were there written records? Were environmental controls done?
. Aw, you're no fun at all! Why not speculate? Where's the harm? I bet you've thought about it. Nencini speculated and so did Mignini, Crini, Matteini, Massei and Micheli. It's allowed so go for it. Give us all a laugh.