Detail of DNA sampling tubes in Laboratory forensic equipment

Understanding DNA Used as Evidence 

Law Offices of Thomas R. Cox III Jan. 6, 2023

The use of DNA in criminal cases began in 1986 with the murder of 15-year-old Dawn Ashworth in Leicestershire, England. A man confessed to the murder, but the details of an earlier murder were so similar to Ashworth’s that police became suspicious and launched a genetic dragnet.  

Using DNA samples from both crime scenes and comparing them with samples from 4,000 men between 17 and 34 years of age in the Leicestershire area, they failed to find a match. A break came when a man was overheard saying he had paid someone else to do a sampling in his name. That man’s DNA was then sampled and turned out to be a match. Colin Pitchfork thus became the first criminal convicted using DNA evidence.  

Since then, DNA evidence has taken on an aura of invincibility, but sampling and the testing process can prove inaccurate and produce false positives.   

If you are facing a criminal charge in or around Dallas, Texas, and DNA sampling is part of the prosecution’s evidence, or you have already been convicted based on DNA evidence, contact the criminal defense attorney at the Law Office of Thomas R. Cox III.   

A former Dallas County prosecutor, Attorney Tom Cox knows the pros and cons of DNA sampling. Also, he can fight on your behalf to challenge the results and use the sampling to help you seek release from prison, depending on which circumstance you’re facing.

Collecting and Testing DNA 

DNA can be obtained from evidence at the crime scene, and the results are compared against those in a nationwide database called the Combined DNA Index System (CODIS). In Texas, police can also draw a DNA sample under a 2019 law targeting suspects charged with 24 qualifying felonies. Generally speaking, in other situations, authorities must seek a search warrant or assert probable cause to take a person’s sample.  

Objects at a crime scene that can contain DNA are many and varied. Of course, anything that has the perpetrator’s blood on it is a prime source, and so is a thread or two of hair left behind. If the victim fought the assailant, the victim’s fingernails might contain the DNA of the assailant. The victim’s body may also contain hair, skin cells, semen, or even blood from the assailant.  

As technology has advanced, it is now possible to obtain what is called “touch DNA,” or low-level DNA. This involves skin cells left behind. Even bruises on the victim’s body may contain the assailant’s skin cells. Any object touched by the assailant may also contain skin cells, including a weapon left behind.  

When a suspect is sampled, it is done through a cheek swab, and in Texas, is then sent to a Department of Public Safety (DPS) laboratory, as are samples found at the crime scene. To ensure the quality of results, DNA testing must be conducted in a laboratory whose equipment and procedures meet stringent FBI standards, which DPS adheres to. 

How Effective Is DNA Testing? 

DNA left at a crime scene can degrade over time. DNA scientists look for 13 “markers” to determine if a sample matches forensic evidence. If all 13 are present, the chance of a false positive is one in a quadrillion. However, if the sample degrades, the possibility of false positives increases. If only five markers are available, the chance of false positives drops all the way down to one in three.  

Like blood samples, DNA samples can be mishandled in the testing process, or even confused with another crime scene or another suspect, though the odds are probably low. An even scarier thought is that it’s altogether possible for someone to plant DNA at a crime scene to implicate another person not involved in the actual crime, perhaps for a revenge motive. 

Fight for Your Rights and Protect Your Freedom 

Cases involving DNA sampling require special attention and consideration. DNA sampling can be your savior if you were convicted without DNA evidence, and your DNA later clears you of the crime for which you were jailed. DNA can also be a huge legal challenge if you’re facing a criminal charge and prosecutors have crime scene DNA evidence that they say matches yours.  

In either case, you’re going to need the counsel and representation of a skilled and experienced criminal defense attorney. If you’re in or around the Dallas area, contact the Law Offices of Thomas R. Cox III. He will work with you to assert your full legal rights and help you challenge the DNA evidence against you, or if you’re already been convicted and serving time, he will search for ways to use DNA to get your conviction overturned.  

Attorney Thomas Cox III also proudly fights for the rights of clients in Irving, Mesquite, Grand Prairie, Highland Park, University Park, and Dallas County.