17 / 8 / 2023

DNA Evidence in Criminal Cases

In recent decades, advancements in forensic science have significantly impacted criminal investigations and legal proceedings. Among these advancements, the use of DNA evidence has emerged as a powerful tool for identifying and convicting persons accused of crimes. DNA, or deoxyribonucleic acid, is the genetic material present in every living organism, and its unique characteristics have reformed the field of forensic investigation in criminal matters. This article explores the fundamental importance of DNA evidence in criminal cases, its applications, and its limitations.

Understanding DNA Evidence

DNA evidence refers to the collection, analysis, and interpretation of biological samples found at crime scenes or on individuals involved in criminal activities. It involves comparing the genetic material found at the scene or on an object with that of potential suspects, victims, or existing DNA profiles in databases. The uniqueness and stability of DNA make it a valuable source of evidence that can link individuals to crime scenes, victims, or each other.

Identification and Exclusion

One of the primary uses of DNA evidence is the identification and exclusion of suspects. By analysing biological samples, forensic experts can establish a match between DNA profiles obtained from the crime scene and those of potential suspects. This can either corroborate or exclude their involvement in the alleged criminal activity. DNA evidence can be instrumental in both convicting offenders but also in exonerating innocent persons.

Cold Case Investigations

Advances in DNA testing techniques, including the use of DNA databases and familial searching, have helped law enforcement agencies revisit unsolved cases. DNA profiles obtained from semen, saliva, or other biological material can be compared with existing DNA databases or used to generate investigative leads. By re-analysing existing evidence or examining new samples, investigators have been able to identify new suspects.

Limitations of DNA Evidence in Criminal Cases

DNA evidence has undoubtedly revolutionized the field of forensic science and significantly impacted criminal investigations. However, it is essential to acknowledge the limitations associated with DNA evidence to ensure a balanced understanding of its role in criminal cases. There remain a number of key limitations identified from case law, in particular. These limitations may be overcome as DNA technology continues to evolve.

Without more, DNA can Only Amount to a Circumstantial Case against a Suspect

DNA evidence is by its very nature regarded as ‘circumstantial evidence’ as it

relies upon an inference to connect it to a conclusion of fact (for example, from

the presence of the accused’s DNA on the knife we can infer that he held it and

used it to stab the victim). Such circumstantial evidence can be clearly contrasted with direct evidence of what a witness saw or observed at the crime scene. Despite DNA evidence often forming part of a circumstantial case, the presentation of DNA evidence presents risks that jurors will attribute undeserved legitimacy to evidence given by a forensic expert. This is commonly known as ‘the CSI effect’.

There is No Reliable Method for Determining How DNA Was Deposited

Another limitation of DNA evidence is the absence of a reliable scientific method to determine how DNA was deposited on an object. Scientists cannot definitively establish whether DNA was transferred directly or through secondary means. The lack of certainty in determining the deposition method can weaken the strength of the Crown’s case and provide opportunities for cross-examination of forensic experts.

There is No Reliable Method for Determining the Age of DNA

Presently, scientists are unable to determine the age of DNA or estimate when it was deposited on an object. The inability to establish the age of DNA can have significant implications for DNA in circumstantial cases. Cross-examination of forensic biologists regarding the timing of DNA deposition has the potential to challenge the Crown’s case and raise doubts about the reliability of DNA evidence.

There is No Reliable Method for Determining the Biological Source of DNA

While there are presumptive tests for identifying certain bodily fluids like saliva, semen, and blood, these tests do not provide definitive proof that the DNA originated from that specific bodily fluid. Establishing the biological source of DNA can be crucial in connecting a suspect to a particular crime. However, the current limitations in determining the biological source of DNA underscore the need for caution when drawing conclusions solely based on presumptive tests.

There are Factors Affecting DNA Persistence

The persistence of DNA on objects is influenced by various factors, such as exposure to sun, water or other environmental conditions. Studies have shown that DNA may not persist after washing or may significantly degrade as a result. These factors further complicate the determination of the timing and manner of DNA deposition and add to the challenges associated with relying solely on DNA evidence.

A Recent Example: Inquiry into Forensic DNA Testing in Queensland

In 2022, an inquiry was announced to ensure transparency, identify opportunities for improvement and ensure public confidence in the collection of DNA and the testing and analysis undertaken in Queensland. The Commission commenced on 13 June 2022 and on 13 December 2022, the final report was provided to the Commissioner for Police and the Premier.

The Commission identified instances of contamination and cross-contamination during the DNA testing process in Queensland labs. This raised concerns about the potential for incorrect results and instances of human error and lax practice in the analysis and interpretation of DNA profiles.

The Commission of Inquiry into Forensic DNA Testing in Queensland underscored the importance of recognising and addressing the limitations and challenges associated with DNA evidence. The findings and recommendations of the inquiry have not only informed reforms in Queensland but have also provided valuable insights for jurisdictions worldwide. It serves as a reminder that while DNA evidence can be a powerful tool in criminal investigations, it must be subject to rigorous scrutiny, adherence to best practices, and ongoing advancements in technology and methodology to ensure its reliability and fairness in the criminal justice system.


While DNA evidence has proven to be a valuable tool in criminal investigations, it is crucial to recognise its limitations. The inability to determine the age of DNA, establish the biological source of DNA with certainty, determine how DNA was deposited, and account for factors affecting DNA persistence are significant limitations that can impact the strength and reliability of DNA evidence.

It is essential for legal professionals, forensic experts, and the justice system to be aware of these limitations and critically evaluate DNA evidence within its contextual framework to ensure a its fair use when presented in criminal proceedings.

Should you or someone you know be charged with an offence involving DNA evidence, it is essential you receive legal advice at any early stage. To discuss your options, call Hugo Law Group in Sydney (02 9696 1361), Canberra (02 5104 9640) or Perth (08 6255 6909) to make an appointment to speak to one of our lawyers.

Damien Mahon, Associate