DNA Evidence in Murder Cases: What Defence Teams Do

How DNA Is Collected at a Crime Scene  

DNA evidence from a crime scene can include blood, saliva, hair, skin cells (touch DNA), or other biological material. In a murder investigation, forensic teams will meticulously search for and collect these traces from surfaces, weapons, clothing, or the victim’s body.  

Crime scene investigators must follow strict protocols to preserve DNA samples. This usually means wearing gloves, masks, and full-body suits to avoid shedding their own DNA onto evidence, and using sterile swabs, containers and tools. Once collected, each item or swab is sealed in tamper-evident packaging and clearly labelled. Maintaining an unbroken chain of custody is vital and every transfer of the evidence (from scene to lab, etc.) is documented so it’s clear who handled it and when.  

Thorough and careful collection is critical not only to capture any DNA present but also to prevent contamination. For example, if two pieces of evidence come into contact or a tool is reused improperly, DNA could transfer between samples. Investigators often photograph and map the scene andchange gloves frequently between handling items. Areas where DNA might reasonably be found (e.g. under a victim’s nails, on a weapon’s grip, on drinking glasses) are carefully swabbed. Environmental factors at the scene also matter. Heat, moisture, or time can degrade DNA, so preservation (like air-drying a wet swab or refrigerating samples) may be needed to ensure a usable profile later. All these meticulous steps are taken to give the best chance of obtaining a clear DNA profile in the lab.  

Despite best efforts, crime scene DNA collection is rarely perfect. Real-life scenes are often chaotic and exposed to elements or bystanders. Mistakes at this stage, like mislabelling an exhibit or improper packaging – can have ramifications later. That’s why an experienced defence team will obtain full documentation of how DNA was collected in your case.  

Was the scene handled according to guidelines?  

If not, that can open avenues to question the reliability of the DNA evidence. A well-prepared defence will scrutinise whether all reasonable measures were taken to avoid cross-contamination and preserve the integrity of the DNA samples from the start.  

Transfer, Contamination and Secondary DNA

One crucial concept when evaluating DNA evidence is DNA transfer. People often ask: ‘can DNA be transferred accidentally?’  

The answer is yes. DNA can move in ways that might surprise those who view it as conclusive proof of direct contact. Primary DNA transfer means DNA is deposited through direct contact (for instance, the perpetrator’s blood on a weapon). Secondary DNA transfer means DNA from a person is carried via an intermediary. For example, imagine Person A shakes hands with Person B, then Person B later handles a knife. It’s possible Person A’s DNA is now on the knife even though Person A never touched it directly. Secondary transfer can potentially leave someone’s DNA at a crime scene even if they were never there.  

Contamination  

Contamination refers to unintended introduction of DNA into a sample, whether at the crime scene, during transport, or in the forensic laboratory. This could happen through an investigator’s own DNA or mixing between samples if procedures aren’t followed scrupulously. An infamous example involved DNA from a lab technician appearing in multiple crime scene samples before labs introduced stringent anti-contamination protocols. In a murder case, if the accused’s DNA is found on, say, the victim’s clothing, the defence must ask: how did it get there? Was it through the crime itself, or could it have been transferred innocently (perhaps via a shared environment) or through contamination at some point after the fact?  

Forensic science recognises that incidental DNA transfer can occur. As a result, the presence of a person’s DNA on an object or at a scene doesn’t automatically mean they were present at the time of the crime or that they committed the act. Context is everything. Did the person have legitimate reason to be there at another time? Is the DNA in a place that indicates involvement, or could it have arrived via secondary transfer?  

Defence lawyers will explore these questions thoroughly. At Olliers, we often work with forensic scientists to conduct what’s called a ‘transfer analysis’ which involves examining how likely it is for DNA to have moved in a given scenario. We will also check if police and lab contamination logs exist, to see if any contamination incidents were noted with the evidence.  

By highlighting the realities of DNA transfer and contamination, a defence team can challenge assumptions that “DNA means guilty” and encourage a jury to consider alternative explanations.  

Partial and Mixed DNA Profiles Explained  

Not every DNA result produces a clear single-source profile. In violent incidents involving multiple people, mixed DNA profiles are common. A mixed profile contains DNA from more than one individual, for example, a weapon handle may carry skin cells from several people. Analysing mixtures is complex: forensic scientists must assess how many contributors there are and attempt to separate the profiles. Where DNA is heavily mixed or degraded, the results can be ambiguous. Partial profiles, where some genetic markers are missing, may further limit certainty. 

A partial DNA profile usually results from a very small or degraded sample. For example, touch DNA may reveal only some of a person’s markers. While partial profiles can sometimes be linked to a suspect, the statistical confidence is usually lower than with a full profile. Probabilities may therefore be far less compelling, a point defence teams examine closely.  

Interpreting partial or mixed profiles often involves subjective judgement and defence lawyers frequently instruct independent experts to review the raw DNA data. An expert can assess whether the prosecution’s interpretation is sound or whether the evidence has been overstated. Some mixtures are so complex they should be considered inconclusive, meaning they cannot reliably include or exclude a defendant. Clearly explaining these issues to a jury can be critical, as incomplete or mixed DNA evidence may be unsafe to rely on as proof of guilt. 

Laboratory Processes and Human Error  

Once evidence arrives at the forensic laboratory, scientists use established processes to extract DNA and create profiles. Labs in the UK are generally accredited and follow quality-controlled procedures. It’s unsettling to consider, but DNA evidence can be wrong due to human error or technical issues. For example, a sample could be accidentally switched or mislabelled, leading to a DNA profile being attributed to the wrong exhibit or person.  

Labs do maintain logs of quality incidents and adhere to the Forensic Science Regulator’s guidelines (in England & Wales) on avoiding contamination and reporting issues. A diligent defence team will probe these lab processes. We might request disclosure of lab records, validation data for any novel techniques used, and previous  cases of errors. We also consider whether the lab followed standard protocols. 

It is rare for DNA evidence to be completely fabricated or wholly incorrect, but “rare” is not the same as impossible. When someone’s liberty is at stake, the defence must leave no stone unturned. By highlighting any lapses in lab procedure or human error, we can challenge the reliability of DNA evidence. Juries need to hear if there’s reason to doubt that a DNA match was processed perfectly.  

How Defence Experts Review DNA Evidence  

An experienced defence team will not take DNA evidence at face value. One of the first steps we take when DNA evidence is central to a case is to engage our own independent forensic expert. Our expert will review every aspect of the prosecution’s forensic findings. This review can include:  

• examining the laboratory’s case file and notes 
• examining the electronic data from the DNA sequencer or analysis software 
• the statistical calculations used to express the match probability 

Defence experts may attempt to replicate the analysis to verify if the same result is obtained. They might even advocate re-testing a sample at an independent laboratory to either confirm the result or check for discrepancies. They will look at whether proper controls were included. 

For mixed DNA, they may test different combinations of profiles to see if the defendant’s DNA really must be present.  

The defence expert essentially puts the prosecution’s DNA evidence under a microscope. Their findings help the defence lawyers decide on strategy: perhaps applying to exclude the DNA evidence if serious failings are identified, or at least to cross-examine the prosecution’s expert vigorously on certain points.  

For example, if the prosecution says “the DNA on the sweatshirt matches the defendant”, a defence expert might point out that it’s actually a mixed profile, where only a partial match to the defendant exists alongside unknown DNA. That nuance can cast the evidence in a very different light in front of a jury.  

It’s also part of our role as defence solicitors to translate all this scientific analysis into plain English for our clients and in court. We ensure the jury (or judge) understands what the DNA can and cannot prove. We might use analogies or visual aids to get across tricky concepts. Ultimately, the goal is to prevent any overstated or unchallenged forensic evidence from unfairly tipping the scales. Especially in murder trials, a robust defence review of DNA evidence is absolutely essential.  

Challenging Prosecution Assumptions  

DNA evidence often comes with assumptions attached. Prosecutors may present a DNA match as if it inherently proves the defendant’s guilt. A key job of the defence team is to challenge these assumptions and remind the court that DNA is just one piece of the puzzle. We craft careful cross-examination questions for forensic witnesses to expose any uncertainty or alternative explanations.  

Another common assumption is that a match statistic (e.g. “one in a billion chance of a coincidental match”) equals proof beyond doubt. We might highlight that the DNA evidence is circumstantial unless it’s directly linked to the act of the crime. For example, a defendant’s DNA on a cigarette butt at the scene might only prove they were there at some point, not that they committed murder.  

The defence also scrutinises how the prosecution has chosen to interpret the DNA results. Sometimes, two experts can look at the same DNA data and draw different conclusions. We ensure that any element of opinion or uncertainty in the forensic evidence is fully aired. If the prosecution’s case “assumes” something like a time frame or method of transfer for the DNA, we will challenge whether that’s scientifically supported or merely speculative.  

In addition, we keep the bigger picture in view. DNA evidence must be considered alongside all other evidence, witness accounts, CCTV, fingerprints, motive, etc. If there’s a mismatch (for example, no other evidence placing our client at the scene except a single DNA trace), we will emphasize that discrepancy. Sometimes DNA might even distract from more plausible leads, perhaps the real perpetrator’s DNA isn’t on the database and the presence of our client’s DNA is coincidental or from an innocent interaction. A specialist defence team is adept at presenting these counterpoints clearly, to ensure that jurors do not give DNA evidence more weight than it truly deserves.  

When DNA Does Not Mean Guilt  

DNA evidence, even if legitimately found and matched to a person, is not the same as proof of guilt. Certainly, a DNA match is compelling evidence that connects someone to a crime scene or object – but context matters. There are many scenarios in which a defendant’s DNA might be presentbut they are not guilty of the crime alleged:  

• Innocent contact: Perhaps the defendant had been in the location or touched the object at a different time completely unrelated to the crime.  
• Secondary transfer: As discussed earlier, the defendant’s DNA might have been carried to the scene by someone or something else.  
• DNA already present: In some cases, evidence used or left by the culprit (a vehicle, a tool, clothing) might already have had the defendant’s DNA on it (if they legitimately used it at another time) before the culprit ever took it. This can happen in domestic settings or among acquaintances.  
• Mistaken identity: If the DNA profile is partial or close to another person’s profile, there’s a slim chance of misidentification, particularly if investigators were already focusing on the defendant.  

Real-life cases illustrate that DNA alone does not guarantee a correct verdict. For instance, the case of Sean Hodgson is often cited: he was convicted of murder in the 1980s but always denied the crime; decades later, advanced DNA testing on preserved evidence proved his DNA was not present and he was exonerated after 27 years in prison. On the flip side, cases like Colin Pitchfork (the first murderer caught by DNA in the 1980s) show how powerful DNA evidence can be in catching the right offender.  

It’s very important for families and defendants to remember that even if forensic evidence seems to point towards guilt, a skilled defence team can often find weaknesses or alternative interpretations in that evidence. At Olliers, we have seen cases where juries, once presented with a fuller picture, understood that the DNA evidence was not as clear-cut as it first appeared. An experienced solicitor can explain to you exactly what the DNA evidence means in your case and what it does not  necessarily mean. This perspective can bring some reassurance in an otherwise frightening situation. DNA evidence does not automatically equal a conviction.  

Courts require the criminal standard of proof, beyond reasonable doubt, and it’s the defence’s job to ensure every doubt about the DNA (or any evidence) is properly explored.  

Why Specialist Forensic Advice Is Essential  

Given the complexities we’ve outlined – from collection procedures and DNA contamination issues to interpreting mixed profiles and potential lab errors – it should be clear why instructing a specialist defence lawyer for a murder or manslaughter case is so important.  

Murder cases are vastly different from more routine criminal matters. They involve “heavyweight” forensic evidence and intense scrutiny. A general criminal lawyer, or the duty solicitor you met at the police station, may have done an excellent job on a past minor case (like a driving matter or a low-level offence). However, they might not have the necessary experience or resources to handle a complex murder case with forensic evidence at its core. In our experience, too many defendants stick with the first solicitor they find, not realising that homicide defence is a highly specialisedfield.  

At Olliers, we understand the tactics of early police investigation, and we know how to engage proactively, for example, by making early representations if DNA evidence looks weak or by ensuring crucial forensic testing is done promptly and properly.  

We also have established relationships with leading forensic experts in DNA, pathology, cell site analysis, etc. This means we can call on the right expert advice quickly. If you instruct Olliers, for example, you’re not just getting a solicitor you’re getting access to a whole network of specialist knowledge built up over decades.  

In practice, this specialist attention can be decisive. We pride ourselves on combining our legal and forensic expertise with a compassionate, supportive approach to our clients. Every case, and every person, matters to us.  

If you or a loved one is facing a murder or manslaughter inquiry, especially one involving DNA or complex forensic evidence, do not leave your fate to chance. Ensure you have a legal team that specialises in these cases. It could be the difference between conviction and acquittal. The stakes couldn’t be higher, which is why specialist advice is not just preferable but essential.