The technician prepares the samples carefully, runs the slides through the computerized sequencer…and one commercial break later, Gus Grissom knows with certainty who killed the Hollywood starlet. It sounds like science fiction, but it isn’t – forensic scientists can literally pinpoint the identity of criminals, down to the genetic level, from the tiniest samples found at crime scenes.

Fans of TV programs such as Crime Scene Investigation and Law & Order know that forensic science is what those in law enforcement use to solve crimes based on available evidence. In fact, forensic science refers to far more than criminal detective work. It is a general term referring to a range of scientific methods used to establish and confirm the facts surrounding any event, place, person or object. Forensic science is used not only in criminal investigation, but civil in litigation as well. Physical sciences such as anthropology, pathology, medicine and even the study of insects (known as entomology) are all used by those working in forensics. Those who study the past from a scientific perspective – paleontologists and archaeologists – have long relied on the tools of forensic science.

Forensics can also be applied to social sciences such as psychiatry and even computer science (for example, recovering data from a hard drive that has been “erased”).

Forensic DNA Screening

One of the most recent developments in forensic science has been the use of DNA screening. Although its application goes beyond identification of the “perp” in criminal cases, this is what many people think of when they hear about “forensic DNA screening.”

One common form of forensic DNA screening that has been around for quite awhile is paternity testing. This of course is used to establish the identity of a child’s father when such an identity is in question.

Recent scientific advances in genetics have made the use of DNA screening much more widespread, particularly in law enforcement and criminal investigations and in the field of medicine when it is necessary to determine familial relationships for clarifying a patient’s health history, finding a suitable donor for a procedure, etc.

How it Works

The use of “DNA evidence” is pointless unless there is something to which such evidence can be compared. The first step is then to collect samples.

DNA, or DeoxyriboNucleic Acid as you may know is the “genetic blueprint,” or “software” that determines the characteristics, or traits of all organic life forms. Passed down from parent to offspring, it is present in every single living cell.

DNA samples can be gathered from anywhere or anything that has been in contact with organic tissues, living or dead. These may consist of:

personal items: toothbrushes, combs, razors, cosmetics, etc.

samples of bodily fluids or tissues: blood, semen, saliva, perspiration, etc.

hair or fingernail clippings

bone, teeth or other remains

Such evidence can even be gathered from fingerprints, since physical contact with virtually anything leaves trace amounts of skin oils and cells behind.

In simplest terms, DNA screening consists of comparing two samples of DNA and looking for any differences and similarities. Today, this primarily consists of studying individual genes, each of which determines a specific trait (such as eye color).

When it comes to studying very old and/or degraded samples (a severely decayed body or severely damaged tissues), scientists use a technique called polymerase chain reaction. A polymerase is an enzyme, or a form of protein that acts as a catalyst which allows DNA to replicate itself. In metaphorical terms, it is similar to using the keyboard command Ctrl-C followed by Ctrl-V in order to “cut and paste” text or other data into a computer document.

The end result of the polymerase chain reaction method is that the scientist is able to amplify a DNA sample to the point at which it can be studied in greater detail.

How It’s Used

One of the first examples of how DNA screening was used to solve a crime involved a case in the small town of Narborough, U.K. over twenty years ago. Two young girls – one in 1983 and the other in 1986 – had been sexually assaulted, murdered and left in the woods. Because the two crimes were so similar, local law enforcement came to believe that both crimes were committed by the same perpetrator. When a local man confessed to one of the crimes, a sample of his DNA was taken from his blood sample and compared to DNA found in semen that had been recovered at the crime scene.

That confession turned out to be false; the man’s DNA did not match that taken from either crime scene. Later however, the police attempted to collect blood samples from all the men in the area in an attempt to find the girls’ killer. A year afterward, a woman reported overhearing a conversation in a local pub in which a man was bragging of how he foiled the police by giving a blood sample for a friend, whom he mentioned by name. The friend, Colin Pitchfork, was later interviewed by local authorities who took a blood sample and successfully matched its DNA to that of the semen samples. (You can learn more about this case in a book titled The Blooding by Joseph Wambaugh [New York: Bantam Books, 1989]).

DNA Screening Today

The most famous case involving DNA evidence in the U.S. in recent years was the high-profile O.J. Simpson case in the mid-1990s. Simpson was accused of murdering his estranged wife and her companion. In the absence of reliable witnesses to the murders, State of California prosecutors built much of their case on DNA evidence gathered from the now-infamous “bloody glove” as well as Simpson’s vehicle. Because of technical difficulties encountered during the laboratory procedures and mistakes made by those handling the items, the jury in the case came to doubt the evidence.

Methods of DNA screening and analysis has come a long way since then, and current procedures are much more reliable.

Today, law enforcement agencies in the U.S. and U.K. collect DNA samples from everyone who is taken into custody and for the past several years have been creating a massive DNA database. While these may ultimately prove to be of great help to investigators, they have raised serious concerns on the part of civil libertarians, the ACLU and other activists over how government and the private corporate interests with which it is closely allied may use – and possibly abuse – this information.