DNA is often collected from a suspect and compared to evidence found at the scene of a crime. The ability to extract and analyze DNA became useful to law enforcement agencies desiring to identify criminal suspects or to prove someone’s innocence. While enhancing investigators’ crime-solving ability, forensic science is not without error and drawbacks.
The science behind DNA evidence
Nuclear DNA (nDNA) is found in various body fluids, bones, hairs and skin cells. DNA is commonly found in shed hairs, skin cells, fluids or on surfaces touched or worn by individuals. The genetic material may be further categorized as Y-chromosome DNA, which is specific to men and passes from father to son. On the other hand, mitochondrial DNA (mtDNA) passes from mothers to their children.
By analyzing DNA samples, sequences and chromosome information, scientists are able to determine if the material came from a man or a woman. The unique sequence is also used to differentiate DNA samples between suspects and victims.
Polymerase chain reaction (PCR) techniques require far less DNA compared to the sample size required for RFLP testing. However, any contamination cancels verification possibilities.
Short tandem repeat (STR) analysis technology assesses specific regions (or loci) on nuclear DNA.
Is DNA testing accurate?
Traditional fingerprinting and eyewitness testimonies are riddled with flaws. Thus, DNA samples derived from crime scenes and suspects are an invaluable part of forensic evidence. In order for DNA evidence to be accurate, investigators must collect samples and handle them appropriately.
Forensic technicians must then use an accepted methodology to analyze the samples. Scientists estimate that there is a one in a billion chance that one individual’s DNA sequence matches another individual’s sample. Thus, DNA forensic evidence is thought to have a 95 percent degree of accuracy.
The potential for error
However, human error has the potential to diminish the accuracy of DNA evidence. If not obtained and handled properly, DNA samples may become contaminated with the DNA of investigators or laboratory technicians. Lab techs must also have the ability to properly analyze samples, which might include material from more than 1 person.
Samples collected might also endure environmental damage through exposure to extreme temperatures or other factors.
Unfortunately, some samples might prove too minuscule in order to provide satisfactory sequencing. Individual states have specific requirements, which if not met regarding the evidence, may lead to objections by defense attorneys and subsequent evidence dismissal by judges.
DNA applications in cold cases
The advancements in DNA technology enhance law enforcement officials to solve older, or “cold case,” files. Early DNA analysis involved a technique known as restriction fragment length polymorphism (RFLP). But the process required large quantities of DNA in samples in order to ensure accurate results. Contaminated samples also prohibit proper analysis. Thus, the method is not a viable option for solving cold cases.
However, PCR and STR methodologies enable forensic scientists to establish profiles based on biological evidence. Clothing, weapons and other objects collected from original crime scenes may harbor cells containing DNA, which is suitable for analysis.
While various cold case files have been solved using DNA evidence, the likelihood of error is always present. Thus, suspects and defense attorneys are wise to exercise a healthy skepticism and carefully weigh the evidence for refutability.