Saturday, April 21, 2018

Newsflash: "Touch DNA" doesn't necessarily require touching

Grits has discussed problems with DNA mixture evidence - particularly regarding so-called "touch DNA" - at some length. But conversations in Texas about the math have ignored an even bigger problem raised in this Wired magazine article: Touch DNA from an individual can be transferred to places to which the suspect has never been. 

The featured case involved an innocent man whose DNA wound up under the fingernails of a murder victim. But the crime was committed while he was hospitalized and could not have committed the offense. It turned out, the same paramedics treated the suspect and responded to the murder scene, somehow transferring his DNA in the process.

This may be more common than anyone - even innocence advocates - have understood. It turns out, for example, about one in five people "walk around with traces of other people's DNA on our fingernails," a study found

Or consider: Scientists have determined that a man who shakes hands with another person then goes to the restroom may end up with their DNA on his penis. Or, according to this Canadian study, a father's DNA may frequently wind up on his daughter's underwear because it "migrates there in the wash." (One wonders how often that latter circumstance may have contributed to false convictions in child molestation cases?)

The emerging questions around touch DNA - both the math surrounding the analysis and the assumptions surrounding what the results mean - are a lot more complex than anyone could have imagined five or ten years ago. Unfortunately, the criminal-court judges charged with sorting out the mess are ill-equipped and unprepared to do so.

Indeed, if these questions are resolved a decade from now, Grits will be pleasantly but seriously surprised. The Trump Administration shut down forensic reform efforts soon after Jeff Sessions became Attorney General, and it's doubtful state-level activities like the review at the Texas Forensic Science Commission can forge national or (really) international standards, which is what's ultimately needed.


Anonymous said...

The National Commission on Forensic Science that was shut down by Jeff Sessions was one of two federal efforts in this area. The NCFS was always the lesser of the two. The more important was (and is) the OSAC, run by the National Institute of Standards and Technology, which is about the development of mandatory standards of practice for technical operations. Those efforts still continue. The OSAC is a scientific effort, as you would expect from NIST which is a scientific organization. There is an involvement of lawyers in the OSAC process, but they are not central to the process.

There is no specific, special math related to "touch" DNA samples. "Touch" DNA (it is a bad term, and should be avoided imho) is simply low-level DNA, which may be either from a single contributor, or from multiple contributors, and may be complete or partial. So the math is the same as for all other DNA samples of those sorts. There are a lot.

The value of a DNA profile in a criminal investigation hinges on the type of biological material it comes from, and the relationship of that biological material to the criminal incident. This is why the serology (body fluid identification) aspect of forensic biology testing remains important. In the case of "touch" DNA, the presumption is that the DNA profile comes from skin cells. That might be a reasonable presumption, but it is not something that could be testified to. There is no test for skin cells. But assuming that the DNA profile comes from skin cells, the question of how the skin cells got onto the item is the critical legal question, which the DNA testing can't answer. Particularly in domestic abuse cases, where the complainant and alleged perpetrator share a living space, there are lots of innocent ways of transferring skin cells over and above the ones you mention. Imagine a fabric-covered chair that is never washed and accumulates over the years the skin cells of people who sit in it. So there will be little value of a "touch" DNA profile on its own (just like there is little value of hair in a domestic abuse case). But it might serve to corroborate the complainant's testimony.

Gritsforbreakfast said...

There's no special math regarding touch DNA, but there IS regarding DNA mixture evidence. And all or nearly all Texas labs were getting it wrong.

Agreed on the "context" issues, but I think you're underselling the math problems related to analyzing mixtures of unknown origin as compared to single-or-two-source comparisons. There, the mathematical models and assumptions underlying them can and do make a big difference, which is why two different black-box products analyzing DNA mixture samples came up with different results in a case in upstate New York. They differed not because the evidence they analyzed was different - the difference was the math applied to it and a dispute among the two vendors about which model most closely reflects reality. Since both are black-box tech, nobody as yet can know the answer. But it does lead a layman to believe the math is in dispute.

Anonymous said...

The link to "The Scale of Misdemeanor Justice" does not work. Gets an error on the library server it attempts to open.

Gritsforbreakfast said...

Fixed it, 9:35, thanks!

Barlow said...

1988, "DNA typing from single hairs"

1998, "Dandruff as a potential source of DNA in forensic casework"

2002, "The propensity of individuals to deposit DNA and secondary transfer of low level DNA from individuals to inert surfaces"

2003, "The effectiveness of protective clothing in the reduction of potential DNA contamination of the scene of crime"

These are only but a few of a slew of scientific papers that discuss non-contact transfer of DNA evidence dating back 30 years or so. And yet there are STILL lab analysts in Texas who are trained to handle biological evidence with out wearing protective face masks, hair nets, or gloves. There are still DNA testing crime labs that don't have a DNA database of their own analysts or CSI people for identification of contamination issues inside the lab.

Dumb, but there is no penalty for being dumb. The dumb still get paid.

PL said...

"Following the staining process, analysts may handle slides on the frosted ends (the area of the slide that may have hospital labels and was labeled by analysts during the inventory process) without gloves in order to transfer them to and from the microscope stage. This handling of the fixed and stained slides is also in accordance with the Institute's documented PPE policy for this specific process."

"They [sexual assault smears] are expressly not an intermediate in the DNA analysis process. All steps in the processing of smears are consistent with this specific and limited analytical purpose. Smears are not processed in a way that is intended to preserve them for later DNA testing. As an illustration, smears are fixed and stained using various solutions. However, none of these solutions are subjected to QC testing to determine if they are contaminated with human DNA. Additionally, the staining of the smears is performed with an acid-based staining solution which will chemically degrade DNA."

Bullsh*t statements made by Dr. Jeff Barnard, Director of The Southwestern Institute of Forensic Science (SWIFS) and, unfortunately, current Chair of The Texas Forensic Science Commission. They are lies to cover-up malfeasance and incompetence.

Barlow said...

So if lawyers simply handle the evidence carelessly and with reckless disregard for proper storage, the evidence can not be DNA tested, according to the CCA. Yet another "out" for the unethical attorneys.

Barlow said...

To go along with the Wired Magazine article...