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The Sketchy Science Behind DNA Sketches

Calcasieu Parish Sheriff's Office / Electronic Arts
Calcasieu Parish Sheriff’s Office / Electronic Arts

Before I go any further, let me say up front that this article is not specifically about a recent break that led to an arrest in an 8-year-old Moss Bluff, Louisiana cold case. By all indications, the DNA-based facial reconstruction was just one factor leading to the suspect’s arrest. The Calcasieu Parish Sheriff’s Office has stated that officers received a tip that led to a DNA profile match, which is an entirely different – and well established – science.

The arrest of Blake Russell in the murder of Sierra Bouzigard seems like great police work all around, and has brought the victim’s family closer to seeing justice served.

That’s a good thing, and the CPSO is to be commended for a job well done.

That said, the whole notion of reliably reconstructing someone’s face based off DNA phenotyping has a lot more in common with The Sims than it does with anything scientific, and the end result is pretty much the same.

Yes, there’s some science behind the faces – but it’s not as clear cut as it seems. Let me explain.

We’re all used to watching exciting cop dramas on TV where crime labs routinely perform scientific miracles, from amazing DNA catches to the timeless “zoom and enhance” scene, when a computer is able to take a blurry black and white photo and somehow create a crystal clear image of the killer reflected in David Caruso’s sunglasses.

CSI Miami Cast Pose With The Stanley Cup
Getty Images

It all makes for thrilling television, but it’s still just fiction. Crime labs don’t work that way. DNA doesn’t work that way. Security camera don’t work that way. And, by the way, tasers don’t knock people out, either. (If you do happen to get knocked out by anything for more than five minutes, you’ll probably wake up with severe brain damage. The real world doesn’t work like the movies.)

However, years of exposure to these sorts of stories make us eager to believe that the crime lab can do just about anything, including building faces from DNA.

But there’s a problem with that.

Sure, DNA markers can reasonably predict certain traits like hair and eye color, but building an entire face from a DNA sample is pretty sketchy science. Sure, you can take a generic face and then apply various predictive markers to it to try to create something that kinda/sorta might resemble the person you’re profiling, but it’s never going to be as accurate as the companies selling this snake oil want us to believe.

Even at its best – if you could get 100% accuracy when creating a face from DNA – you’d still be left with something that doesn’t quite match up to reality, for a variety of reasons.

First, there’s no way to take environmental factors into account. Hard living takes a toll, after all. Work outside in the sun all your life, and your skin leathers up. Grow up poor and malnourished, and your bones and teeth are going to form differently. Eat more or eat less, and your body, along with your face, will change accordingly. And we’ve all seen those terrifying before-and-after pictures of what Meth can do to a face.

Not even once, kids.

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Furthermore, there’s a common belief that our DNA is like a fingerprint – because, for the purpose of identifying matching samples, it kind of is – but that’s only because DNA varies wildly from unrelated person to unrelated person. It’s why DNA matches are always presented with a degree of probability. There’s a whole formula for calculating it, but it all comes down to determining how likely it is that the DNA sample from a crime scene could have been left by someone other than the suspect. In a good DNA match, that likelihood is very, very improbable.

There are decades of legal precedent regarding the use of DNA as a sort of genetic fingerprint, and it’s pretty reliable. However, our fingerprints don’t change much over time. Our DNA, on the other hand, does.

When you take a sample of DNA and try to build a face out of it, the science tends to fall apart, because we don’t actually have the same genes from the day we’re born until the day we die. Our genetic makeup is constantly mutating as we age and our cells divide. In fact, our cells get around one genome change for every three divisions, and when you consider that about 50 million of our cells die and are replaced each day, that adds up pretty fast.

Of course, most of these mutations are benign and don’t usually have any significant effect on our bodies, but over time, each division does take a little chunk away from the telomeres that act as a kind of safety net and buffer for our chromosomes as they pull apart during each division.

Telomeres are like little caps that protect the ends of our chromosomes from deteriorating or fusing with neighboring chromosomes whenever our cells divide. When division occurs, sometimes the dividing chromosome will get cut short, which would have a pretty damaging impact on our genome if bits of our DNA started getting lopped off every time our cells divide. The telomeres sit at the ends of our chromosomes and keep this from happening, since any chunk that gets cut off will come from the telomere rather than the chromosome itself.

USDOE / Human Genome Program
USDOE / Human Genome Program

The problem is that our telomeres get shorter each time a little slice of them gets cut during cell division. Over time and after enough divisions, they run out – which is when when we start noticing the effects they’ve been protecting us from all our lives.

It’s called the Hayflick Limit, or cellular senescence. Think of a chromosome as a knight inside a shining suit of telomere armor – the armor protects the knight each time he gets hit – but, over time, bits of the armor start falling off, leaving the knight inside exposed to the business end of a bad guy’s sword. That’s the Hayflick Limit.

With the telomeres depleted, each division starts chopping off a bit of the delicate chromosome until, eventually, our cells start undergoing bigger and bigger replication errors that start causing all kinds of changes in our bodies.

We call it aging. As our genome begins to change, our hair starts to turn grey, our joints turn arthritic, our eyesight deteriorates, etc… This happens at different rates for different people, depending on any number of factors that either accelerate or slow telomere decay. It’s why some people I went to high school with are either grey or bald already, while I still have a luxurious head of dirty blonde Chia Pet hair.

The point is that DNA can’t ever accurately reconstruct someone’s unique face. There are just too many variables and too much about our environment that can affect our appearance that our DNA has no way of “knowing” about.

It’s science fiction, folks. It sounds good and we’d all like to believe it, but it’s just an expensive fantasy.

Calcasieu Parish Sheriff's Office
Calcasieu Parish Sheriff’s Office

To sum all this up for the too long / didn’t read crowd, DNA facial reconstruction isn’t all it’s cracked up to be. Its only function is as a potential tool to help law enforcement narrow down a pool of suspects – which is nothing to sneeze at, but is hardly the amazing new scientific breakthrough the companies that sell this kind service like to make out.

In the Bouzigard cold case, the CPSO says the facial reconstruction led officers to change the ethnicity of their suspect, which helped lead to an arrest – but determining ethnicity from DNA doesn’t require facial reconstruction, and the snapshot provided from the DNA sample doesn’t even look much like the suspect who was arrested. (Not to my eyes, anyway.) This sort of “evidence” also isn’t likely to be admitted in court, due to just how questionable (and ultimately unreliable) the science behind it is.

However, as the CPSO has shown, when used properly – as just one of many tools law enforcement can use to narrow a pool of suspects – there is value in it. To ultimately secure a conviction, you’ll still need corroborating evidence, such as the DNA match the CPSO says they have in the Bouzigard case, then you’ll have to do things like establish motive and opportunity, and all that other good stuff we have prosecutors for, but a facial reconstruction based off DNA should never lead directly to a conviction.

Because this is real life, not Star Trek CSI.

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