Forensics for Writers: Part 7

A Throwback Article

This article was originally published at the Mad Genius Club in April 2016. If you’ve made it this far, there’s only one essay left on this topic! Um, in this series, anyway… I make no promises.

The Gold Standard

Next week will be the wrap-up, but as I said last week, DNA needed a post all it’s own. I’m going to get more into the speculative here, this is a fascinating and rapidly changing field when it comes to any aspect of science, not just forensics.

DNA (deoxyribonucleic acid, and isn’t that a lovely thing to say ten times fast) is considered the gold standard because of two things. One is based on math - the probabilities of two people matching across 13 alleles is something approaching billions if not trillions to one. With this in mind, if we have that full loci match, we can assuredly say that this person shed/bled/left this bit of DNA at the crime scene. Which isn’t to say that they did it, but I’ll get into that in a bit. The other reason this is the gold standard is television. The TV shows may get 90% of their stuff wrong, but this doesn’t matter to the general public. And the jury is drawn from the general public. The result is that juries almost demand there be DNA evidence presented to them.

With modern technology, DNA can be extracted, amplified, and sequenced from a single skin cell. Even hair, which unless it has been yanked out and retains the follicular tag, has no nuclear DNA, contains mtDNA and can be used to narrow down the identification of a suspect or a victim. The problem, of course, is finding that single cell to work with. DNA, like any other evidence, has drawbacks. For one thing, although the cost is falling and the time to run tests is going down with it, it’s still not a cheap or easy test to run. A skilled technician is needed to run the process, and interpretation of the results isn’t always clear-cut.

Imagine this. There’s a crime scene, and it’s been swabbed for DNA and tests are being done. We’re talking swabbed down to the single-cell collection point, and it’s a house where, say, drugs were being sold. Random people in and out all the time. The investigator had tried out a new shampoo and it made his scalp react and he’s got dandruff. The first officer on scene sneezed. The victims were stabbed and staggered through the house leaving bloodtrails everywhere until they died on the kitchen floor. A young woman and her dog (don’t kill the dog in a story. Readers hate that). So we have contamination, commingled blood from two humans (at least, the killer(s) cut himself) and a dog, and oh, yeah, it’s August, they didn’t have air conditioning and it must be 40 deg C in the house by the time they were found. At that temperature, DNA starts to break down. Or, wait, even worse - they got out to the end of the backyard and died in the little creek back there. Water. Water is not the forensic scientist’s friend.

Pipetting samples for gel electrophoresis, the old-school way of looking at DNA.

While I know I could separate out the dog’s DNA from the humans, interpreting commingled DNA is a challenge, and that first uniform who sneezed (vomiting would almost be better because then the later investigators would have known about it. The sneeze was barely noticed by the guy, and he certainly didn’t think to tell anyone) meant that there will be an unknown profile in the samples near the side door where the attack initiated. There will also be profiles from people who were in the house recently. It’s going to be complicated, messy, and expensive. In a murder investigation, this isn’t really a factor, but in a lesser crime like a robbery, no department is going to this level to investigate.

DNA is not cut-and-dried, and as you write your story, this can be either a good thing or a bad thing. Good because we aren’t yet at a level of technology that allows every crime to be solved simply by scanning DNA at the scene and proclaiming ‘Eureka! ‘Twas him!” within minutes. Even when on-site DNA testing is possible - and it’s coming - there will still be all the complications I touched on in my scenario, plus more I couldn’t work in there. I’m not downgrading the on-scene investigators, I’ve sat in classes with some of them, been taught by another, but the reality is that they aren’t trained for everything that I’ll be trained for, and I won’t be trained in some aspects of their job (although the other half of my degree is Investigation). The lab and the field aren’t the same. Even if you’re writing science fiction in the future, complete detection, interpretation, and analysis of DNA is going to remain more complex than waving a magic wand. Although there’s an interesting story idea there using Clarke’s Law...

And the bad thing about DNA can actually be helpful to the writer. Introducing conflict, and tension, by having DNA not come back usable can be useful. Your investigator will have to work harder. Your villain will have to try to be sneaky in weird ways as they attempt to leave no DNA behind them... which will mean they miss something by over-focusing on that. Writers dream up hyper-intelligent criminals, but in the real world most of them are doofuses, to steal a word from Peter Grant (I always enjoy the Doofus of the Day posts when I catch them on his blog). We don’t need DNA to convict the doofuses of the world, but as I said above, juries expect it. When the proceedings go to trial, which doesn’t happen much of the time.

DNA may not be the miracle drug of the forensic world, but it does come close. I’ve enumerated some of the flaws, but it remains the most individual and solid of evidence in a case. In a few short decades, we have come from needing a lot of DNA material (blood or semen, usually) to being able to accurately process a profile from touch DNA - the shed skin cells left on a doorknob, for instance. I know I said that I don’t think there will ever be a magic wand, but I do think that with a trained, skilled operator, that wand might start to approach magical. I also think that public perceptions of DNA and what it can be used for, which might not be accurate, may drive policy and legislation in unexpected directions.

DNA databases are the other half of this tool. Once you have a profile, what do you do with it? From the TV, and books about crime, I suspect most of you are already familiar with CODIS. What some of you may know, and something that has implications for us as people as well as writers, is that the Supreme Court has ruled that anyone arrested for a major crime can have a DNA sample taken and entered into the database. Arrested - not convicted. Above and beyond the implications of this, are the growing privately held databases of DNA that are being created for genealogy purposes. What if your investigator is given access to one of those? I’m sure there are contracts between the company and the individuals who send in their samples to be told how much of a Neanderthal they are... surely there is some kind of privacy shield in place. People are too savvy to just let their DNA be stored anywhere on any old server (reached in and shuts off internal snark button) Ahem. Where was I? Oh, yes... could your investigator, armed with a warrant and Truth and Justice on his side, gain access to these private databases?

What about the movement to have everyone sampled at birth? What if this goes on?

And with that, I shall leave this topic. Next week, I’ll wrap up and answer any lingering questions.