It was 1989, and I was teaching forensic science in Seaford H.S. There was a murder of a thirteen year old girl in a neighboring community. The victim’s body was found stuffed in a sleeping bag in her neighbor’s basement, ready for disposal. A young man, living in the house, was accused of the crime, and the high profile trial ensued.

It was one of the first trials on Long Island to present DNA evidence, attempting to prove beyond the proverbial shadow of a doubt, that the defendant was guilty. Expert witnesses were called to explain what the technology of DNA analysis was. The presentation was so complex, and the defense attorney so adept at clouding this testimony, the jury ignored the DNA evidence, and convicted the defendant on the weight of the bite mark evidence and a partial palm print. Palm prints are accepted as being as reliable as individual fingerprints. This evidence is visual and easy to grasp.

Since then the term DNA has become part of the popular vocabulary of the day, one TV commercial even suggesting that you can transfer your DNA to your Smart Phone, the ultimate personalization.

There are many misconceptions about DNA, and even more about the “analysis” of DNA used in criminal investigations. I would like to share, and hopefully, simplify what I’ve learned about this topic from my more than twenty years teaching at Seaford H.S. and Hofstra U.

Let me dispense with the misconceptions.  First, the term “DNA Fingerprint” is meant to suggest that evidence derived from DNA is as reliable as that of a true fingerprint left by a human hand. Second, the analysis techniques do not take material from fingers.

DNA is a specific chemical compound located in most body cells. It enables the biochemical instructions for making new cells and their functioning proteins to be passed to those new cells. DNA has two functions, coding the important chemicals, and having the capacity for storing lots of information. If you think about this, you will realize that the code must contain instructions for both; what makes for a functioning human, or what makes us the same, and then, what makes us all individual and different from each other.

In forensic science, evidence in a criminal investigation is best if you can move its status from representing a class of individuals, to belonging to a specific individual. This means that in DNA analysis, we are looking for that part of the CODE that makes us unique.

The state of the current technology allows us to find “markers” in the code. These “markers” don’t code for specific proteins, but vary in frequency from one individual to another. Sounds like a winning technology, right? Well, almost.

This current technology employs two important components. The first isolates the specific marker, allowing us to link the DNA to a group. This isn’t any better than finding a defendant to have type O+ blood. To make the analysis significant requires us to find several markers for the defendant. Like the ABO blood groups, these markers have statistical probability in the human population. By using the laws of statistics which link the statistical occurrence of each marker mathematically, we can narrow the probability of our suspect’s “marker profile” belonging to anyone else. It is even possible to boil the probability down to one in seven billion, which is the human population. One in seven billion is the probability for human fingerprints. The term “DNA fingerprint” comes from this statistical analysis.

Another simpler form of DNA analysis, called PCR, determines a person’s “DNA type” in a complex protein profile. It does not have the same statistical probability as the previously discussed form of analysis. Because of this, it is only used to eliminate a suspect, not to convict.

The prevailing wisdom is that nothing supersedes a fingerprint as individualizing evidence. As you can see, we have one human, one finger, one fingerprint. Studies have not found two individuals with the same fingerprint. With DNA analysis, sometimes called “DNA fingerprinting” we have one human, one marker, one group of humans, then statistical analysis.

In subsequent articles I would like to further clarify the various types of DNA analysis.

I invite your questions.