Biology Magazine

Q and A: Could the Sensitivity of Modern DNA Testing Prove Confusing in a Contaminated Crime Scene?

Posted on the 19 June 2011 by Dplylemd

Q: I attended your forensics panel at SINC OC and remember you saying that DNA can be extracted from a single cell found at the scene. Is that correct? I have a meticulous serial killer that I want to leave only DNA clues but (for me at least) that begs two questions: How far is the one-cell method of DNA ID developed now and wouldn’t the DNA results become muddied since many persons would have been in the same area? In other words, how would friends/family/acquaintances be ruled out or in as suspects when they most likely would have physical contact with the victim?

P.I. Barrington, Riverside, CA

A: The techniques that allow very small DNA samples to be useful are well-established and have been the last 15 years. They are the polymerase chain reaction (PCR) and short tandem repeats (STR). Together they are referred to as PCR/STR. The PCR technique basically copies the existing DNA chains so that a single DNA chain–therefore a single cell–can be used to produce as much DNA as is needed. Since the copying is exact, all the DNA produced by this method is identical to the original DNA strand. This process is called amplification but it is basically a duplication of the existing strand. The short tandem repeat is simply a method of analyzing the DNA and producing a profile from multiple short segments of the target DNA.

What this all means is that a very small DNA sample, and theoretically only a single cell, can be used to generate a DNA fingerprint.


Q and A: Could the Sensitivity of Modern DNA Testing Prove Confusing in a Contaminated Crime Scene?

Your other questions bring up a very difficult problem that will be an increasing problem in the future. If the DNA techniques are so sensitive, what do we do about extraneous DNA found at the scene? Since people shed skin cells all the time, a busy public place could theoretically house the DNA from thousands of people. But as with the blues, context is everything.

If the crime scene DNA is found in a drop of blood or a smear of semen or a fingerprint, the DNA found in that sample would belong to the person that left the sample behind. Could it be contaminated by other DNA? Of course, but this contaminant would be in very small amounts. In addition, the extraneous DNA might belong to a family member or friend or someone who had a reason to be at the scene before or after the murder. That’s not always true in the case of the killer. Often he has no innocent reason for having deposited his bodily fluids or fingerprints at a murder scene.

So let’s look at a scenario such as this: the killer does his deed. He washes his hands in the sink. He uses a hand towel to dry his hands. The crime lab technicians evaluate the towel and find DNA present. The DNA proves to be from several people. The victim, the victim’s spouse, the victim’s children, and maybe the victims next-door neighbor who visits daily. But another DNA is found. One that cannot be matched to any known individual. Later a suspect is identified and indeed this DNA matches him.

What does this evidence tell investigators? It tells them what any evidence does. That the individual identified by the DNA had contact with that towel. That’s it. It doesn’t say anything else. This is true of all evidence. It merely serves as a link between a person and another person, place, or object. Your investigators must then uncover the circumstances under which this person’s DNA was left on that towel. If he can prove he had been there for dinner the night before and had indeed washed his hands then this evidence is of little value. But if he swears that he doesn’t know the victim and has never been in the victim’s home, that’s an entirely different story. Again, context is everything.


Q and A: Could the Sensitivity of Modern DNA Testing Prove Confusing in a Contaminated Crime Scene?

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