Hey IQuestion,
Some background for my answers below:
"The majority of the DNA under Rebecca Zahau's fingernails was her own," said Grubb. "Various fingernails were tested as separate samples and one of them showed a DNA mixture but the level of DNA was so low that it was an un-interpretable mixture."
In addition to the fingernail sample, unidentified DNA also was recovered from the rope used in Zahau's alleged hanging; a large knife used to the cut the rope; the bed frame to which the rope was tied; a door knob on the balcony door; and a pair of black gloves found on a table in the mansion, Grubb said.
[SNIP]
Because the amount of mixed DNA recovered was so minuscule, San Diego County Sheriff Bill Gore said it was unnecessary to collect DNA samples from Zahau's boyfriend, Arizona tycoon Jonah Shacknai, or Jonah's ex-wife, Dina Shacknai.
http://www.760kfmb.com/story/160680...ne-some-evidence-not-tested?clienttype=mobile
Both the questions and the answers between the press and Grubb were not sufficiently detailed to give a clear picture of what the situation with the mixed DNA is.
A look at the actual lab reports would be nice, and in some cases the public is fortunate enough to be able to see those, but the only time I have seen lab reports has been from cases that have gone to trial. For example, I have the DNA lab reports from the Anthony, Zimmerman and Knox cases.
I can speculate a bit on what I think is going on in this case.
I believe that the DNA was tested using conventional/standard testing methods.
What is needed in this case, and I don’t think happened, is to have the DNA that was deemed “low-level” and or “uninterpretable” to be tested using more aggressive techniques, techniques usually employed only in private DNA labs and extremely rarely in law enforcement labs. (The New York City Office of Chief Medical Examiner Department of Forensic Biology is the only LE facility that I’m aware of.)
http://www.nyc.gov/html/ocme/html/hss/hss_home.shtml
Trace DNA analysis which has many critics and detractors, (and I’m one of them,) does have a legitimate use as a tool in developing leads, its use in the court system, however, is where I believe EXTREME caution needs to be employed. (Its use in the court system in California received a legal setback as an appellate court ruled against it. The case was People of California v. Hector Espino)
If for example, LCN methods were employed on the difficult samples in the Zahau case and it was learned that one of the persons of interest as identified in the civil suit could not be ruled out of one or more of the DNA mixtures then it would certainly lend a great deal of validation to the theory of murder rather than suicide. It would, (or at least should) be the catalyst needed for further investigation.
I don’t know if further testing is possible by a private lab at this stage or whether the samples, because of their small size, were consumed completely by SDSO testing.
It should be noted that DNA mixture interpretation is notoriously and often nightmarishly difficult and does not make for a “good day at the office” for lab techs.
DNA mixture analysis falls more within the realm of art rather than science and mistakes are QUITE possible.
Regardless, given the evidence in this case supporting homicide, I believe that all available DNA methodology should have been employed to determine if some of the areas in the crime scene were contacted by people that should not have been there.
If a person fended off multiple attackers, would scrapings under the fingernails yield "MIXED DNA"???
Yes, but an innocent explanation is also possible. It’s more important to see if certain persons of interest can be identified as potential donors to the mixture.
Would a closer forensic study be able to separate out each?
People could be ruled in or out as possible contributors to a given sample. Legally bordering on useless but, as I’ve said, valuable in developing leads, and the steering the investigation in the correct direction.
Would the person testing, be dumbfounded/dismiss the findings because they assumed there was only one assailant?
Impossible to answer without more details. Confirmation bias may have driven the analysis of complex DNA samples to a result consistent with the case context the analyst was given.
Here’s an excerpt from an interesting article on mixture analysis that most people find to be an eye-opener.
Dr Dror's and Dr Hampikian's experiment presented data from a real case to 17 DNA examiners working in an accredited government laboratory in North America. The case involved a gang rape in the state of Georgia, in which one of the rapists testified against three other suspects in exchange for a lighter sentence, as part of a plea bargain. All three denied involvement, but the two DNA examiners in the original case both found that they could not exclude one of the three from having been involved, based on an analysis of swabs taken from the victim.
As is almost always true in forensic-science laboratories, these examiners knew what the case was about. And their findings were crucial to the outcome because in Georgia, as in many other states, a plea bargain cannot be accepted without corroborating evidence. However, of the 17 examiners Dr Dror and Dr Hampikian approached—who, unlike the original two, knew nothing about the context of the crime—only one thought that the same suspect could not be excluded. Twelve others excluded him, and four abstained.
Though they cannot prove it, Dr Dror and Dr Hampikian suspect the difference in contextual information given to the examiners was the cause of the different results. The original pair may have subliminally interpreted ambiguous information in a way helpful to the prosecution, even though they did not consciously realise what they were doing.
And DNA data are ambiguous more often than is generally realised. Dr Dror thinks that in about 25% of cases, tiny samples or the mixing of material from more than one person can lead to such ambiguity. Moreover, such is DNA's reputation that, when faced with claims that the molecule puts a defendant in a place where a crime has been committed, that defendant will often agree to a plea-bargain he might otherwise not have accepted.
This one example does not prove the existence of a systematic problem. But it does point to a sloppy approach to science. According to Norah Rudin, a forensic-DNA consultant in Mountain View, California, forensic scientists are beginning to accept that cognitive bias exists, but there is still a lot of resistance to the idea, because examiners take the criticism personally and feel they are being accused of doing bad science. According to Dr Rudin, the attitude that cognitive bias can somehow be willed away, by education, training or good intentions, is still pervasive.
http://www.economist.com/node/21543121
