How to Use Touch DNA as Forensic Evidence in Your Crime Fiction Novel
You’ve probably heard of touch DNA in forensic cases in the news or on your favorite crime drama, and now you’re thinking that maybe you could use it in your crime fiction novel.
But first, do you know what touch DNA is? Is it different than “regular” forensic DNA? Are there case/crime scene situations that are better for this type of forensic evidence than others? Is there a specific way to use it as evidence in your book?
As a former forensic DNA analyst, I can answer all these questions and help you make sure your books are accurate. In this post, I explain what touch DNA is and how it differs from other sources of forensic DNA. I also provide a list of 11 helpful things to know when using it as forensic evidence in your crime fiction books.
Skin cells have high turnover and are shed continuously as you go about your day. As a result, you are leaving behind pieces of yourself everywhere you go.
Touch DNA is DNA that’s deposited on a surface after someone touches or handles an item and leaves skin cells behind. You naturally leave DNA on the clothing you wear as it rubs against your skin, as well as your computer and cell phone and other items that you touch.
If you want to use touch DNA in your crime fiction novel, here are 11 things that you should before you do.
DNA is Present at Lower Levels Than Other Body Fluids
If you read this post on the likelihood of getting forensic DNA profiles from certain body fluids, you’ll see that touch DNA is ranked as “low.” This means that the probability of getting good DNA results is highly variable.
Touch DNA is tricky. Sometimes you get a profile; sometimes you don’t. And if you do get a DNA profile, it might not be a good one. There’s a higher likelihood of it being a low-level mixture, which can make interpretation difficult.
Don’t Mix Touch DNA with Another Body Fluid
Remember, touch DNA is at the bottom as far as getting a good DNA profile is concerned. If you mix your touch DNA evidence in with another body fluid, it will essentially be “drowned out.” There are skin cells there, but the other body fluid will overwhelm the touch DNA, and the person who contributed the touch DNA won’t be detected.
What this means for your books is this: don’t mix blood and touch DNA together. You will only get the DNA from the person who contributed the blood. The same goes for touch DNA and saliva, touch DNA and semen, and so on.
Make Sure the Touch DNA Evidence (and All Forensic Evidence) in Your Books is Probative
Forensic evidence is only probative if it actually proves something. A good guideline to follow to ensure your evidence is probative is to ask yourself if the DNA can be “explained away.” If it can, then it’s probably not probative.
For example, trying to use touch DNA as evidence a suspect’s own house or any other place he/she has been before isn’t probative. It’s expected that his or her DNA would be present. The same goes for items left behind at different locations.
Perhaps a hat is left behind at a murder scene that has the suspect’s touch DNA on it. Excellent! He or she was at the scene! But… if the suspect is a friend or an acquaintance of the victim or can somehow show that the hat got there by noncriminal means, then it’s not probative.
Another example is swabbing the victim’s arm in an assault case for touch DNA of the suspect because the victim said he grabbed her. It’s probative if he denies knowing her, but if they know each other and he’s denying the assault, the suspect can say he casually touched her arm and didn’t hurt her. One way to make the touch DNA in this type of case more probative would be to get a touch DNA profile from the bruised neck of a strangulation victim. People don’t casually touch each other’s necks and leave visible marks.
Location matters as far as probative DNA evidence is concerned. You can put an item in two different locations and have it be probative in one situation and not probative in the other. Touch DNA on tools in the backseat of a burglary suspect’s car isn’t probative. It’s his car, and the tools belong to him. Of course his DNA is on them. But finding those same tools with his DNA on them at the scene of a crime is. It places him at the scene.
Make Sure to Rule People Out if Necessary
You need to rule out other people if you find a touch DNA profile that might have other people’s DNA on it. Swabbing the steering wheel of a stolen car to see if the suspect’s DNA is present is a great idea, but you need to rule out the owner of the vehicle. The same idea goes for any other stolen item.
Do Not Use Touch DNA from a Public Place
Collecting touch DNA from a public place or an item that many people have handled is just an all-around bad idea (unless there are extenuating case circumstances to warrant it). There’s no way to rule everyone out. Some examples are stores/banks, money, and hotels.
An example of an extenuating circumstance would be an armed robbery where the suspect touched the teller counter and left visible smudge marks (but no latent prints) and there’s no other forensic evidence.
Be Sure to Use An Item that Will Produce Good DNA Results
During my time in the crime lab, I performed a study on which touch items produce better DNA results, and I found specific patterns. Certain factors affect the success rates of touch DNA.
Factors that contribute to higher success rates are items with non-porous and rough surfaces and handling the items for a longer period of time. Examples include gloves, hats, steering wheels in cars, and personal possessions. Items with smooth surfaces and “one touch” items usually do not produce good DNA results. Examples include external car door handles, small metal objects (keys), and touching a surface for a short time.
The environment can also play a factor. Heat, humidity, and the elements can all interfere with the DNA process. Any item found outside is not a good touch DNA item unless it’s protected from the elements somehow.
The Type of Case and Other Evidence Make a Difference
Not all labs accept touch DNA evidence, and they don’t always process it if they do. Every crime lab has submission polices that outline the evidence they will accept for certain types of cases.
Evidence is prioritized based on the type of case, likelihood of getting results, and probative value. Touch DNA evidence in a property crime case that has other items with blood on them probably wouldn’t be examined. But a lone fired cartridge case from a sniper, a touch DNA item in a homicide with no other evidence, or fragments of a bomb in an explosives case that killed people probably would.
Touch DNA Results Can Be Different for the Same Items
You may hear about different touch DNA results for similar items. Technical approaches to touch DNA (and therefore the results) in different crime labs vary widely. Some crime labs have special protocols that deal with low-copy DNA. Others don’t. The decision on how to approach touch DNA evidence is up to each individual lab. Low-copy DNA is complicated, and not all labs choose to do it. In my old lab, we processed touch DNA evidence, but we used the same technical approach for all body fluids.
Crime labs (and law enforcement agencies) also vary in how touch DNA is collected, extracted, and amplified, all of which can affect DNA results. You probably wouldn’t worry about this too much in your crime fiction books, but it’s good to know in case you see instances of mixed success for touch DNA testing for similar item types and aren’t sure how to use that item in your book.
Some People Shed Touch DNA Better Than Others
People who shed high numbers of skin cells are considered “shedders.” There’s really no way to officially know if someone is a shedder unless they participate in a study.
Be Aware of Secondary Transfer
Indirect, or secondary, transfer involves touch DNA passing through an intermediary before it’s deposited on a surface. For example, I shake hands with someone with clammy palms. Then I go touch an item and transfer that person’s DNA to that item instead of my own. Indirect transfer could be a fun plot twist in your book!
You Can’t Use Touch DNA in Your Books Pre-2000s
The first study describing touch DNA was published in 1997. The reason that forensic scientists can get DNA profiles from miniscule amounts of DNA today and they couldn’t before is because of a technique called the polymerase chain reaction (PCR). PCR acts like a DNA photocopier, producing millions of copies of the DNA.
Touch DNA has been common in crime labs in the last 10 years or so. I worked as a forensic DNA analyst for 7 years, from 2007 to 2014. When I first started, we processed some touch DNA evidence, but not a lot. The last few years I was there we received considerably more touch DNA items (usually from property crimes). Some crime labs get so much touch DNA evidence that they have specific rules limiting its submission. (See #7 above.)
We’ve made it through the list, and now you should know what touch DNA is, how it’s different than other types of forensic DNA evidence, and some guidelines for using it in your books.
Is there anything else about touch DNA that you’d like to know? Ask it in the comments below!