What is the most important point to keep in mind when working with forensic DNA evidence? There are probably a lot of answers to that question, depending on your experience and perspective. I'll let you in on my opinion for now, as a former DNA forensic scientist. One of the imperatives of working forensic DNA cases in this modern age is this: accepting that there are cases (many, in fact) that DNA can't resolve.
Almost every time I testify, I'm asked "why might you not find DNA?" This is a good question, one which I usually answer with a fairly long list of possibilities, but it all boils down to three main points. 1. DNA may not have been deposited in the first place. Does this mean that the incident didn't happen as reported by the victim or witnesses? Not necessarily - more on that a bit later. 2. Maybe too little DNA was deposited for the lab to test and identify. But can't you guys detect even a few cells? More on that, later, too. Or, 3. Perhaps DNA was deposited at an adequate level, but much of it was washed away or degraded over time. I saw a special on cold cases solved by DNA decades later - I don't believe there's anything you can't do. Well, keep reading.
1. DNA may not have been deposited in the first place.
We've become so accustomed to DNA evidence being presented in criminal justice cases that we seem to need to take a collective step back to reflect on a case in which it just isn't there. It really depends on the scenario and the knowns of the case what this lack of DNA on an evidence item could mean.
The murderer doesn't always cut herself on the knife and leave drops of her blood at the scene. The burglar may have kept his gloves on throughout the entire crime, never touching anything with a bare hand. A child molester doesn't always leave semen evidence for us to test.
And, of course, DNA may not be present on a tested evidence item simply because the scenario didn't unfold the way investigators believed or the witnesses stated or the victim recalled. Corroborating DNA evidence with reported scenarios is a tricky business, one which doesn't always result in a resolution tied up with a big red bow.
2. Maybe too little DNA was deposited for the lab to test and identify.Remember the days when crime labs couldn't get DNA from anything smaller than a blood drop the size of a quarter? And remember when, even when they started getting DNA from smaller samples, the odds of someone else having the same DNA profile was only one in several thousand? Well, I don't - that was before my time.
But I was there for the early years of the current DNA typing technology, Short Tandem Repeats (STRs). Those were the days in which we tested mainly blood, semen, and saliva. We had a good idea of what we could and couldn't get results from and we ended up with a lot of single-source DNA profiles. These result in straight forward comparisons to reference samples which yield either an exclusion, if the profiles don't match, or a match. In the case of a match, we calculate and issue some crazy-big statistic that illustrates to the reader (the investigator or attorney or juror) just how significant this match is (spoiler alert: the number is often in the quadrillions - matches are pretty darn significant). And as great as this is and was, the criminal justice system wanted (and in many cases, needed) more.
Science over time, I've found, rarely disappoints. The techniques and products that result from years of experimentation, trial and error, grant funding and academic research end up being a culmination of the best approach among many.
Instead of changing the sites we used for forensic DNA typing, researchers found that we could extract and clean up the DNA a little better and attain higher sensitivity. They modified the primers and added a few more. They improved the reagents that we used to get our profiles and made them a little more robust. They made instruments that could automate sample processing so that we could do more samples in less time. All of this has led to higher throughput and more sensitive results.
Currently, scientists are not just attempting to get DNA profiles from well-defined body fluid stains, as before, but also from areas of evidence items that have tested faintly positive for a body fluid. They are swabbing areas of items that someone in the case may have touched. These types of samples have much, much fewer cells than, say, a fat drop of blood. And, while significant to the case and incident at hand, these samples are likely to contain not only very few cells, but mixtures of more than one person's DNA, further complicating the analysis.
3. Perhaps DNA was deposited at an adequate level, but much of it was washed away or degraded over time.It's important to remember that DNA is a molecule, one with millions of parts. Cells must be intact in order to properly preserve the DNA. And hundreds of cells must be present in a sample in order to obtain a decent profile for comparison. Wiping or washing a surface can remove cells. Environmental factors such as heat, UV light, or bacteria can break cells open, exposing DNA and ultimately breaking it down.
Also, it's useful to know that the laboratory process, itself, is lengthy, requiring many phases, none of which perfectly preserve all of the DNA in the sample from one step to the next. If I detect 200 cells in a sample in the lab, it doesn't mean that 200 cells were originally deposited on the evidence item at the time of the crime. Even in the best evidence-preservation scenario, there is loss of genetic material on the crime-scene-through-laboratory-testing journey.
In the end, as much as I love forensic DNA (and I hope you do, too), it's important to keep its limitations in perspective in every case. The presence of DNA evidence does not prove guilt. The absence of DNA evidence does not prove innocence. The current state of forensic DNA technology is, however, amazing! I think we can all relish in that without abandoning our role as critical thinkers.
Megan Inslee spent 13 years as a DNA forensic scientist in Washington State. She has her Bachelor's in Biology as well as her Master's in the Genetics track of Laboratory Medicine from the University of Washington. She currently resides on an island outside of Seattle with her husband and three small children, writing technical documents, preparing grant proposals, and providing consultation on a freelance basis.