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DNA Evidence: What Patrol Officers Need to Know

Properly collecting and processing new smaller DNA samples is often the key to solving crimes.

February 03, 2011  |  by David Spraggs

There's no question that forensic DNA evidence allows police to solve crimes that would otherwise go unsolved. Before its use, forensic biological science was limited to blood typing and other tests that couldn't identify an individual contributor of the genetic material. Now many people take for granted the ability to link a specific person to a crime scene.

As you probably know, DNA stands for deoxyribonucleic acid. Every person, except for identical twins, has a unique DNA profile or genetic fingerprint. Professor Sir Alec Jeffreys developed the scientific basis for DNA fingerprinting in the mid-1980s.

It's this unique fingerprint that allows forensic DNA to identify specific individuals that may have contributed DNA evidence to a crime scene. The advent of DNA analysis has revolutionized criminal investigations, and new techniques continue to improve the ways in which it can be used.


Of course, DNA is only useful in finding suspects if it can be matched to specific individuals. That's why the DNA Identification Act of 1994 provided the statutory authority to create a national DNA database. The FBI oversees the national forensic DNA program, which is called CODIS, the Combined DNA Index System.

There are two primary types of DNA profiles stored in the national DNA database. The first are convicted offender and arrestee profiles. Each state has its own laws regarding the arrest and conviction requirements that allow an offender sample to be uploaded into the national DNA database. Many states are passing laws to allow DNA profiles to be collected at the time of arrest, not after conviction. This is a good thing and will lead to many more cases being solved through DNA. The second type is forensic profiles. These are DNA samples that have been recovered from crime scenes and also from missing persons.

The CODIS Website provides striking statistics regarding the significance and success of this technology. According to an FBI administered Website, as of November 2009 the national DNA database contained more than 294,027 forensic DNA profiles and 7,635,428 convicted offender and arrestee DNA profiles. All 50 states have contributed to the database, which has assisted more than 100,000 investigations by providing law enforcement agencies with more than 101,900 hits or DNA matches.

Trace Evidence

DNA technology has evolved during the 13 years I've been a detective. First, the sample size required for DNA analysis has gotten smaller and smaller. No longer do we need to collect an entire drop of blood or large semen stain. Today, sample sizes are measured in nanograms of genetic material-just over a billionth of a gram is needed to generate a complete DNA profile. This minute sample size has allowed for the advent of trace or touch DNA collection.

Touch DNA is the process of collecting trace amounts of DNA from specific items of evidence. This technique is used in all types of cases, including property crimes. This is perhaps the most significant advancement in forensic DNA technology that I've seen during my career.

Solving Cases with Touch DNA

I worked a sexual assault a few years ago in which touch DNA helped solve the case. An unknown male had raped a young woman. There were no witnesses and no real physical evidence was recovered from the scene.

The victim's body and clothing is the primary crime scene in most sexual assault investigations, but the sexual assault examination didn't reveal the presence of semen on the victim's body. When the state forensic laboratory examined the sexual assault kit, only the victim's DNA profile was recognized-no male DNA was found. This was not surprising because the suspect had a difficult time maintaining an erection and he didn't ejaculate.

In this case the suspect had briefly kissed the victim's upper chest/lower neck area. Knowing this, a prudent patrol CSI swabbed the victim's neck with sterile cotton swabs moistened with distilled water. The officer feared that this transitory biological evidence would disappear once the victim put on clothing before being transported to the hospital. It turns out that these swabs provided the suspect's full DNA profile-the only piece of evidence in this case to do so.

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