With the advent of DNA (deoxyribonucleic acid) technology and especially with its introduction into forensics in the 1980’s, revolutionary results have been obtained. DNA genetic profiling has become an integral part of many cases that fall into the Medical Examiner’s jurisdiction and the legal system as well. Whether it is a biological specimen (blood, tissue etc.) or evidence collected at a crime scene, Forensic Biologists can identify a biological source and associate the DNA to a victim, suspect, or crime scene evidence. Even if other tests have failed, DNA can identify a possible source to a reasonable degree of scientific certainty or exclude a suspected source. The power of discrimination utilizing DNA can be extremely high and provides a powerful investigative and legal tool.
DNA is the building block for all living systems. Every nucleated cell of the human body contains DNA. From an individual, various tissues and blood contain identical DNA types. No two people have the same DNA with the exception of identical siblings: therefore, DNA evidence collected from a victim or crime scene can be linked to, or eliminate a victim or suspect. In sexual assault cases for example, biological evidence such as hair, saliva, blood, semen, skin cells etc. which are collected from a victims’ body can be compared to the DNA of known suspect samples to include or exclude a suspect.
In recent years, it has become customary to conduct post conviction DNA testing when probative biological evidence is present, in order to confirm or refute the guilt of individuals convicted of crimes. Oftentimes, no other means of forensic testing are available and without DNA testing, an innocent individual could continue serving years of imprisonment.
The Tarrant County Medical Examiner’s state of the art Forensic Laboratory was established in 1990 and continues to provide economical and timely services to more than 100 different law enforcement agencies. In addition to serological methods which confirm the presence of biological material, techniques which include Short Tandem Repeat (STR), greatly improve turnaround time and utilize the very small sample size that are often seen in forensic case work. Additionally, the laboratory also offers Y-STR analysis which targets only male DNA. This is useful in cases in which only a very limited amount of male DNA is present or in mixtures which contain mostly female DNA. Possible sources of DNA evidence include, but are not limited to:
· Bite marks or saliva transfers on the body – saliva
· Fingernail scrapings – blood or skin cells
· Surface of used condom – semen or skin cells
· Bed linens – semen, sweat, hair or saliva
· Clothing – hair, semen, blood, skin cells or sweat
· Cigarette butts, toothpicks, drinking glasses or cans, chewing gum - saliva
· Dental floss – saliva, skin cells or semen
· Tape or Ligatures – hair, saliva or skin cells
In addition, the Tarrant County Medical Examiner’s DNA laboratory is a designated CODIS (COmbined DNA Index System) Laboratory. CODIS, also referred to as the DNA database, is the FBI (Federal Bureau of Investigation) computer system that links all 50 states, Puerto Rico and other countries and is utilized by government laboratories. It provides a means of electronically comparing forensic casework profiles at a local, state and national level to convicted offender profiles generated by each state, and to forensic casework profiles generated in other case working laboratories within and between states. These database searches may result in solving cases which have no other association, or link unsolved cases to a single perpetrator, which can provide additional investigative leads.