Issues with the use of DNA Evidence in Criminal Cases


Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from a crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.



Blood testing:

Forensic genetics aims to identify the origin of a biological sample. Forensic testing started when Karl Landsteiner discovered human ABO blood groups. With blood groups, elimination of suspects based on their blood type became possible. For example, if a suspect had type A blood, but type B blood was left at the scene by the perpetrator, then the investigators could rule out the suspect. Unfortunately, the reverse is less helpful. If the suspect and perpetrator both had the same blood type, investigators couldn’t concluded the guilt of the suspect. Blood identifiers later evolved into 8 distinct categories which improved the ability to eliminate suspects but still couldn’t conclusively identify a suspect (Jobling and Gill).

DNA Testing:

The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA. Minisatellite patterns in DNA create what we know today as the DNA fingerprint. The DNA fingerprint can specifically pinpoint individuals with a higher accuracy rate than previous methods (Augustyn et al.). Around the same time, scientists discovered a breakthrough for victims of sexual assault. Bodily fluids of both the victim and the assaulter can mix during an attack. A new process called differential lysis could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill).

Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This method became the first DNA-based method used in a criminal investigation. The first case used minisatellite patterns to link two murder homicides from several years apart. The DNA evidence then exonerated an innocent man, and implicated the guilty party when he tried to evade DNA testing. His guilt became reinforced when his DNA profiles matched the murderers, and he gave a full confession (Jobling and Gill).

Finding DNA at a crime scene can’t guarantee that you’ll find a murderer. Issues with non-pure samples and limited samples make DNA work difficult, and accreditation of facilities and new techniques can take time (Jobling and Gill).


Subjectivity of DNA Interpretation:

Subjectivity might influence lab based forensics in complex cases. For example, not all DNA found at a crime scene comes from a single individual. The sample could include a mixture of several individual’s DNA. A study published in 2011 looked specifically at this issue (Dror and Hampikian).

The Case:

After a gang rape in Georgia, investigators used a mixture of DNA to prosecute one suspect after another suspect took a plea bargain to testify against him. Forensic scientists can look at DNA mixtures and conclude one of the following: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little possibility that their DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that their DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded from the DNA mixture. To the jury, this meant that the forensic examiners concluded the suspects DNA matched DNA in the sample (Dror and Hampikian).

The Study:

The 2011 study gave DNA evidence from this case, including DNA profiles of the suspects, to 17 forensic examiners. These examiners had no context of the case, and they worked independently. Of the 17, only 1 came to the same conclusion as the original examiners who knew the context of the case. Twelve of the 17 concluded that the prosecuted and convicted suspect had no relation to the DNA found in the sample, and the remaining 4 deemed the test inconclusive. This not only shows that subjectivity occurs due to the context of the case, but the 17 examiners had three different conclusions based on their past experiences. This proves that not all DNA analysis is objective (Dror and Hampikian).

Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but this isn’t currently required by the US (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence.

False positives, on the other hand, are matches between sample DNA and test DNA that result from issues in the forensics lab. In multiple cases forensic labs have accidentally switched victim and suspected rapist’s DNA leading to wrongful convictions. For other cases, forensic scientists misread or misunderstand a sample. In a case from 1993, a man became wrongly convicted of rape after someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so jurors can make accurate judgements based on DNA evidence (Thomas et al.).



DNA analysis is a useful tool for the criminal justice system, but possible subjectivity raises issues in forensic testing. To combat wrongful convictions, organizations like The Innocence Project use DNA testing to free the innocent (“Help us put an end to wrongful convictions!”). But, forensic testing standards need reformation in order to prevent subjectivity and to ensure proper education of juries.



Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018,

Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004.

“Help Us Put an End to Wrongful Convictions!” Innocence Project,

Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455.

Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.