Author Forensic Question: Planting DNA Evidence

Question:

I’m considering writing a crime novel that involves the antagonist framing others for crimes he has committed. He is a genius level sociopath who studies his victims' habits by analyzing their trash.  His day job is with the local trash company (handy for him.)

Now the question. Can he use semen from a condom (if it's not too old) to plant on/in a victim? He rapes, kills, and then plants the evidence along with other clues that lead to his intended second victim?

Amryn says:

This is a great scenario for a novel and would certainly throw the police off for a while. For the antagonist to pull this off, he will need to take some precautions. First of all, if he rapes the victims himself, he’ll most certainly need to wear a condom. Secondly, how he kills his victims will be important in determining if he’s left any of his DNA behind. For instance, if he strangles them with his bare hands, it is possible his DNA could be found from a swab of the victim’s neck. The use of a knife would run the risk of cutting himself and leaving his own DNA behind that way.

As far as planting the evidence, that’s certainly possible. In fact, the reverse of this has been done in real life. A man was put in prison for rape, largely because of the DNA evidence against him. While he was in prison, he placed some of his semen in a ketchup packet and smuggled it out to a female friend. She then planted the semen on herself and said she’d been raped. When they collected the evidence and processed it, the DNA came back to the man who was in prison—a pretty good alibi for the time of the rape. He insisted that the DNA results must have been wrong in the first place or someone else had his DNA profile. Of course, his scheme didn’t work, and as far as I know, he’s still in prison. All that to say, yes, planting that sort of evidence is definitely possible.

If your antagonist takes the condom from someone else’s trash, he runs the risk of having another person’s DNA present as well. When things are thrown away, DNA from several sources (presumably everyone who lives in the house) will come in contact with other objects and transfer will happen. There might not be significant enough transfer to matter, but it could result in the bad guy inadvertently transferring the male’s DNA as well as his partner’s DNA to the victim. If that’s his intention, there’s no problem, but it could also provide a way for your hero to figure out that something just doesn’t seem right about this.

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Amryn Cross is a full-time forensic scientist and author of romantic suspense and mystery novels. Her novel, Learning to Die, is available on Amazon. The first book in her latest series, loosely based on an updated Sherlock Holmes, is available for pre-order on Amazon. Look for Warzone in January 2015. You can connect with Amryn via her website, Twitter and Facebook.