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Injustice Anywhere is a Threat to Justice Everywhere
Serving Southwest Minnesota
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Injustice Anywhere is a Threat to Justice Everywhere
Jurors hear this line and, in an instant, the case feels over.Years of television, podcasts, and true-crime documentaries have trained us to believe that a DNA match is the gold standard of proof. It sounds scientific. Certain. Objective. If DNA is unique to every person, then a “match” must mean the person was there, touched the object, committed the act. Case closed.Except that is not what it means. Not scientifically. Not legally. And not in the way crime labs actually generate DNA results.
One of the most important things a jury can understand—and one of the most misunderstood ideas in modern criminal trials—is the DNA match meaning. Because when a prosecutor says “match,” they are using a word that does not appear in most DNA reports, does not reflect how the testing was done, and often does not accurately describe what the results show.
So what does a DNA match prove?
Far less than most people think.
DNA testing does not tell a story. It does not recreate events. It does not show who touched what, when they touched it, or why they touched it. It does not show whether a crime occurred. It does not even show that a person was present at a particular time. What DNA evidence shows—at its core—is that genetic material was detected in a sample and that, under certain assumptions, a person’s profile could be included as a possible contributor.
That is a very different statement from: “The defendant’s DNA matched the evidence.”
And yet, in courtrooms across the country (including Minnesota courtrooms), that shorthand phrase is repeated by prosecutors as if it carries the weight of certainty.
This gap between what DNA testing actually shows and what jurors believe it shows is where serious misunderstandings happen. It is also where innocent people can become entangled in cases built on powerful-sounding but deeply misunderstood science.
Because modern DNA testing is extraordinarily sensitive. It can detect microscopic amounts of skin cells—sometimes called touch DNA or trace DNA—left behind by casual contact or transferred indirectly through everyday interactions. A handshake, a shared surface, a piece of clothing passed from one person to another can all move DNA without anyone realizing it. This is known as secondary transfer, and it happens far more easily than most people realize.
Then there is the reality that most crime scene samples are not clean, single-source samples. They are DNA mixtures, containing genetic material from two, three, sometimes four or more individuals. These mixtures cannot be “read” like a fingerprint. Instead, they are interpreted using complex statistical software such as STRMix, which produces something called a likelihood ratio—a statistical estimate, not a declaration of identity.
At no point in this process does a machine print out: “Match.”
Instead, the result is a probability statement about whether a person could be included as a contributor to a complicated mixture of DNA, based on a series of assumptions built into the software and the analyst’s interpretation. That nuance is often lost by the time the testimony reaches the jury.
This is why understanding DNA evidence explained in plain language is so critical. Because once you peel back the courtroom phrasing, you begin to see how much of a “DNA match” is really an interpretation layered on top of statistics, assumptions, and extremely small amounts of genetic material that may have arrived at a location in entirely innocent ways.
That is how people end up facing criminal charges based on DNA even when they have never been to the crime scene. That is how jurors hear certainty where the science speaks in probabilities. That is how the word “match” can carry a weight it was never meant to hold.
In this article, you will learn what a DNA match actually means in a criminal case—and what it does not mean. You will see the critical difference between presence and behavior, why touch DNA and transfer complicate nearly every case, how DNA mixtures and probabilistic software shape modern testing, what likelihood ratios really say, and why these factors have played a role in wrongful convictions built on misunderstood DNA evidence.
Because when you understand how DNA testing truly works, you start to realize something surprising:
A “DNA match” is often the beginning of the conversation, not the end of it.
If your Minnesota case involves DNA evidence, it helps to understand how labs interpret results and how those results can be challenged.
For a deeper dive into forensic issues, visit our Forensic Law / DNA evidence page.You can also read related posts that expand on key issues in this article:
touch DNA & low-template DNA problems,
challenging DNA evidence in court,
and 10 DNA questions to ask your attorney.
When most people hear the phrase DNA match, they picture a scene straight out of television: a pristine sample, a blinking computer screen, and a scientist announcing with confidence that the DNA from the crime scene belongs to one person and one person only. In this version, DNA functions like a biological fingerprint—clean, singular, and definitive. The story told on TV is simple: investigators find DNA, they compare it to a suspect, and the system returns a clear answer. This is the popular understanding of what a DNA match is and the DNA match meaning most jurors carry with them into a courtroom before a single witness testifies.
In real criminal cases, however, DNA evidence rarely looks like this. Crime scene samples are often messy, degraded, partial, or mixed with genetic material from multiple people. Instead of a single, clean profile, labs are frequently working with complex data that requires interpretation. Rather than producing certainty, the testing process produces statistics. Analysts do not declare identity; they calculate probabilities. They assess whether a person’s genetic profile could be included as a contributor to a sample under a series of assumptions about how many people may have contributed DNA and how the data should be interpreted. This is a very different reality from the television version, but it is the reality presented in courtrooms every day.
The problem is that jurors do not hear the statistics the way scientists intend them to be heard. The word match carries enormous psychological weight. In ordinary language, a match means equivalence—two things that are the same. If two puzzle pieces match, they fit perfectly. If two fingerprints match, they come from the same person. So when jurors hear that DNA “matched,” they often translate that word into certainty and identity, even if the testimony they heard was actually describing a statistical inclusion, not a definitive identification. The shorthand language collapses the nuance of probabilities into the simplicity of sameness.
What many people do not realize is that “match” is not a scientific term used in most DNA reports. Laboratories typically use phrases like “cannot be excluded,” “consistent with,” or provide a likelihood ratio expressing how much more likely the DNA results are if a person contributed to the sample versus if they did not. By the time this careful scientific language reaches the courtroom, it is often summarized into the far more powerful—and far more misleading—word “match.” Understanding this gap is the first step in understanding why the common perception of a DNA match is so different from what the science actually supports.
At its core, what DNA evidence shows is remarkably simple: a laboratory detected genetic material in a sample and was able to compare portions of that material to known profiles. That is the starting and ending point of the science. DNA testing identifies the presence of biological material—usually skin cells, saliva, blood, or other trace matter—not the story of how it arrived. This is the critical distinction in the debate over DNA presence vs contact. The presence of DNA on an item does not automatically mean a person touched it, handled it at a specific time, or played any role in a criminal act. It means only that their genetic material was detected in that location.
What DNA testing cannot tell you is often more important than what it can. It cannot tell you when the DNA was deposited. It cannot tell you how it was transferred. It cannot tell you who physically placed it there. And it certainly cannot tell you whether a crime occurred at all. Those questions fall outside the limits of molecular biology. Yet in court, the discovery of DNA is frequently treated as if it answers these questions. The leap from “DNA was found” to “this person did this act” is not a scientific conclusion—it is an assumption layered on top of the laboratory result.
Understanding the difference between contact, ownership, and presence is essential. Imagine DNA found on a door handle. That could mean the person opened the door yesterday, last week, or months ago. It could mean they shook hands with someone who later opened the door. It could mean they handled an object that then brushed against the handle. Now consider DNA on clothing. That clothing may have been shared, washed together with other garments, or transferred through ordinary handling. The same is true for shared objects like phones, tools, or furniture. In each example, DNA evidence shows presence of genetic material—not proof of contact at a particular moment, not proof of ownership, and certainly not proof of criminal behavior.
Minnesota takeaway: In many Minnesota prosecutions, the key fight is not “was DNA detected,” but what the DNA can actually support—presence, not behavior—and whether the state is overstating that distinction.
Touch DNA—sometimes called trace DNA—refers to the tiny amounts of skin cells that naturally shed from our bodies and settle onto the surfaces we contact. You do not have to bleed, sweat, or leave visible material behind. Simply handling an item, brushing against a surface, or even being in close proximity can deposit microscopic genetic material. Modern forensic testing is so sensitive that it can detect just a few cells. That sensitivity is scientifically impressive, but it also means labs are now testing material that is easily moved, easily shared, and easily misunderstood.
Because so little material is needed, the question in many cases is no longer whether DNA can be found, but how it got there. This is where secondary transfer DNA becomes critical. Secondary transfer occurs when DNA moves from one person to an object through an intermediate step. A simple handshake can transfer skin cells from Person A to Person B. If Person B then opens a door, picks up a tool, or handles clothing, Person A’s DNA may now be present on those items—even though Person A never touched them. In some situations, even tertiary transfer can occur, where DNA passes through multiple people or surfaces before being detected.
Consider everyday scenarios. Two people share a living space. They sit on the same couch, use the same remote control, handle the same dishes, and wash their clothing together. DNA can move freely through these shared environments. Laundry is a particularly effective vehicle for transfer: clothing tumbling together in a washer and dryer can exchange skin cells between garments. The same is true for shared towels, bedding, or frequently handled objects like phones and keys. None of this requires criminal conduct; it is simply how trace biological material behaves in the real world.
These realities create one of the most significant risks of wrongful inference in criminal cases. When jurors hear that a person’s DNA was found on an object, they often assume direct contact at the time of the crime. But with touch DNA, that conclusion may be completely unsupported. The presence of genetic material may reflect ordinary, innocent interactions that occurred hours, days, or even weeks earlier—or interactions that occurred indirectly through other people.
This is why understanding DNA can be transferred is so important. Touch DNA does not come with a timestamp, a chain of custody for how it moved, or an explanation for why it was deposited. It is simply there. Without careful explanation, the leap from “DNA detected” to “this person handled this object during the crime” can happen quickly in a jury’s mind. And that leap is not grounded in the science of how trace DNA actually behaves.
Related reading: The problems with touch DNA and low-template DNA in criminal cases.
A DNA mixture is a sample that contains genetic material from two or more people. Instead of a single, clean DNA profile that can be read from top to bottom, the lab is looking at overlapping genetic signals layered on top of one another. This is what experts mean when they talk about a mixed DNA sample. The data does not arrive pre-labeled by contributor. It is a composite of markers from multiple individuals, and the challenge is figuring out which pieces might belong to whom.
Most crime scene samples are mixtures. That surprises many people because television rarely shows it that way. In the real world, objects are touched by multiple people before, during, and after an alleged crime. A door handle, a piece of clothing, a steering wheel, a phone, or a tool may carry genetic material from everyone who has handled it over time. Add in first responders, medical personnel, investigators, and lab technicians, and it becomes easy to see how quickly a surface can accumulate DNA from several sources. By the time a swab reaches the lab, it is often a biological collage rather than a single-source sample.
The difficulty with a DNA mixture is that the lab cannot directly observe how many contributors are present. That determination is itself an interpretation. Analysts look at the number of genetic peaks at various locations in the profile and make a judgment call: two contributors, three contributors, sometimes more. That initial assumption becomes the foundation for everything that follows. If the number of contributors is off, the entire interpretation can shift. Yet this step is rarely appreciated outside the forensic community, even though it plays a critical role in how a mixed DNA sample is ultimately reported.
Separating contributors in a mixture is not like sorting colored beads into piles. DNA markers overlap. Some people share the same genetic markers at certain locations. Some markers are faint, degraded, or partially obscured by others. Analysts must decide which signals are meaningful, which are noise, and how to group them into possible contributor profiles. These decisions are guided by training and protocols, but they still involve judgment. There is no visual way to confirm that a particular set of markers definitively belongs to a particular individual within the mixture.
Because of this complexity, the interpretation of DNA mixtures involves a series of assumptions layered onto the data. Assumptions about how many people contributed. Assumptions about which peaks are significant. Assumptions about how to treat faint or ambiguous signals. Each assumption narrows the possibilities, but each also introduces the potential for error or disagreement. Two analysts looking at the same mixed DNA sample may reasonably differ on how it should be interpreted, especially in close or degraded samples.
This is where the science begins to move beyond direct human interpretation and into statistical modeling. Rather than claiming to separate the mixture by hand, modern labs increasingly rely on software to evaluate the data. These programs use mathematical models to assess how likely it is that a particular person’s DNA could be included as a contributor to the mixture. This shift reflects an acknowledgment that mixtures are too complex for simple visual interpretation.
The introduction of probabilistic genotyping software is a response to this challenge. Instead of declaring a “match,” the software evaluates thousands of possible combinations of contributors and produces a statistical result. It calculates how much more likely the observed data would be if a specific person contributed to the mixture versus if they did not. This approach is powerful, but it also means the result is no longer a direct reading of the evidence. It is a probability statement built on top of the initial assumptions about the mixture.
Understanding how a DNA mixture is explained in court requires appreciating just how much interpretation happens before a result is ever reported. What may sound like a straightforward conclusion to a jury is actually the end product of layered judgments, statistical modeling, and complex data analysis. Without that context, the phrase “mixed DNA sample” can be mistaken for something simple, when in reality it is one of the most complicated forms of forensic evidence presented in criminal trials.
Related reading: What the Noblis DNA mixture study means for your case.
When DNA evidence involves a complex mixture, crime labs increasingly turn to specialized software rather than relying solely on a human analyst. One of the most widely used programs is STRMix. STRMix explained in plain terms is essential to understanding how modern DNA results are generated in courtrooms today. STRMix is a form of probabilistic genotyping software. It does not “see” DNA the way a person might read a chart. Instead, it uses mathematical models to analyze the raw data from a mixed DNA sample and calculate statistical probabilities about who could have contributed to that mixture.
The reason software like STRMix is used is because DNA mixtures have become too complicated for reliable manual interpretation. When multiple contributors overlap at the same genetic locations, the human eye cannot confidently separate which markers belong to which person. There may be faint signals, degraded peaks, or shared genetic markers that create ambiguity. Rather than risk subjective interpretation, labs input the data into software designed to run thousands—sometimes millions—of simulations to evaluate possible contributor combinations.
This is where the process shifts from observation to modeling. STRMix does not identify a person’s DNA in the sample. It evaluates how well a person’s known DNA profile fits within the possibilities suggested by the mixture. It considers assumptions entered by the analyst, such as how many contributors are believed to be present, and then applies algorithms to calculate statistical outcomes. The result is not a declaration of identity but a numerical value known as a likelihood ratio.
A likelihood ratio in DNA evidence is often misunderstood in court. In simple terms, it compares two scenarios: how likely the DNA data would appear if the person contributed to the mixture versus how likely it would appear if they did not. For example, the software might produce a result stating the DNA findings are 10,000 times more likely if the defendant contributed to the mixture than if they did not. That sounds powerful, but it is a comparison of probabilities—not proof that the person’s DNA is definitively present.
This distinction is critical because a likelihood ratio is not the same thing as a “match.” It does not say the DNA belongs to the person. It does not say they were at the scene. It does not say they touched the object. It says that, under a set of assumptions, the data fits better with one scenario than another. That nuance is easily lost when results are summarized in courtroom testimony with phrases like “consistent with” or, more misleadingly, “matched.”
Another layer of complexity is what many refer to as the black box problem. STRMix and similar programs are proprietary software. The underlying source code is not publicly available, and defense experts often have limited ability to examine how the algorithms function internally. The inputs and outputs can be seen, but the internal workings—the precise way the software reaches its conclusions—are largely shielded from scrutiny. This creates understandable concern in a legal system that depends on transparency and the ability to test evidence through cross-examination.
Because probabilistic genotyping relies on both human assumptions and complex algorithms, questions arise about reliability. The analyst must decide how many contributors to assume, how to treat ambiguous data, and what parameters to enter into the software. The software then processes those choices and produces a statistical result. If the assumptions are off, the output can change significantly. This is not a flaw in the mathematics; it is a reminder that the results are only as sound as the inputs and interpretations that guide them.
These concerns are why courts examine DNA software evidence under standards for scientific reliability, often referenced through Rule of evidence 702. When looking at Rule of Evidence 702, the key issue is whether the method is reliable, properly applied, and accurately explained to a jury. The danger is not that probabilistic genotyping exists—it is that its complexity can be oversimplified in testimony, giving jurors the impression of certainty where the science is actually speaking in probabilities.
When jurors hear that STRMix was used and a large likelihood ratio was produced, it can sound like a technological confirmation of guilt. In reality, it is a statistical opinion generated by software interpreting a complex mixture through a series of human-guided assumptions. Understanding probabilistic genotyping in this way helps clarify why the result should be viewed as one piece of evidence to be carefully examined, not as a definitive statement that a person’s DNA “matched” the crime scene.
One of the most common pitfalls in DNA evidence presentation is the prosecutor’s fallacy, a logical error that can easily mislead a jury. At its core, the fallacy occurs when a probability about the evidence is mistakenly interpreted as a probability about the defendant’s guilt. This occurs when the probability of evidence conditioned on a proposition is mistaken for the probability of a proposition conditioned on evidence. The million ratio represents the probability of observed evidence given two mutually exclusive propositions. It does not represent the probability that either proposition is true.
Suppose a crime lab reports a likelihood ratio (LR) from a mixture using probabilistic genotyping:
“The DNA is 50,000 times more likely if Defendant contributed to the mixture than if Defendant did not.”
What this means:
Transposing the conditional (the fallacy):
A prosecutor says to the jury:
“The probability that this DNA came from Defendant is 1 in 50,0000. They must be the source.”
Why this is wrong:
Realistic forensic example:
Statistics in DNA cases are often miscommunicated in other ways as well. Analysts may produce likelihood ratios or express the probability that a DNA profile would appear if the defendant contributed versus if they did not. These numbers are nuanced, conditional probabilities, but in the courtroom they are frequently simplified or translated into language that implies certainty. Phrases like “this DNA matches the defendant” or “cannot be excluded” are easily misinterpreted by jurors who are not trained in statistics. The subtle differences between probabilities, statistical models, and inclusion language can be lost entirely, creating a perception that DNA results are infallible.
Jurors naturally expect clear answers. When a prosecutor presents DNA evidence with confident language, jurors often hear certainty where none exists. The brain is wired to favor definitive conclusions, especially in high-stakes situations like criminal trials. Complex explanations of likelihood ratios, mixtures, and probabilistic genotyping are difficult for laypeople to digest quickly, making the shorthand of “match” or “cannot be excluded” far more compelling. This cognitive bias can turn an inherently probabilistic result into a seemingly absolute statement of guilt, even when the science only indicates a possible inclusion.
The danger of phrases such as “cannot be excluded” is particularly pronounced. While scientifically accurate, this wording can be misleading in practice. Saying a defendant’s DNA “cannot be excluded” from a sample is not the same as saying their DNA is present or that they were at the scene. It is a conditional statement that allows for multiple possible contributors. Jurors, however, often hear it as a confirmation that the defendant’s involvement has been proven. This misinterpretation can amplify the weight of the evidence far beyond what the science justifies, particularly in cases with trace or mixed DNA samples.
Compounding this issue is the emotional and psychological context of a trial. Jurors are faced with a high-stakes decision: determining guilt or innocence. When DNA evidence is presented, they may assume it is the most reliable form of evidence available. The combination of technical jargon, misinterpreted statistics, and the persuasive language of prosecutors creates an environment in which even well-intentioned jurors can misunderstand the limits of DNA testing. They may assign near-absolute certainty to results that are, in truth, probabilistic and dependent on multiple assumptions.
Understanding the prosecutor’s fallacy in DNA cases is critical for both legal practitioners and the public. Defense attorneys often emphasize the difference between probability and proof, clarifying that statistical results are not direct evidence of criminal behavior. By educating jurors—or readers—about how DNA statistics can be misstated, the risks of misinterpretation, and the conditional nature of phrases like “cannot be excluded,” it becomes possible to evaluate DNA evidence more accurately. Recognizing these nuances helps ensure that DNA is considered appropriately in the context of all other evidence rather than treated as an unassailable verdict.
Related reading: How cognitive bias can affect forensic science.
Even though DNA evidence is often treated as the gold standard in criminal cases, it is far from infallible. In fact, wrongful convictions based on DNA evidence have occurred in multiple high-profile cases, highlighting how even scientifically sophisticated evidence can be misinterpreted or overstated. One of the most common ways an innocent person ends up implicated is through a chain of factors that, individually, might seem minor, but together can create a powerful illusion of guilt. These factors include DNA transfer, mixtures, software interpretation, and misrepresented statistics.
The first factor is transfer. DNA can be left on objects, clothing, or surfaces without a person ever having been at the scene of a crime. This occurs through secondary transfer—such as shaking hands with someone who later touches a doorknob—or through shared spaces and objects, like furniture, phones, or laundry. In these situations, an innocent person’s DNA may appear on an object simply because it passed through another person or environment, not because they committed a criminal act. Modern forensic testing is sensitive enough to detect microscopic amounts of material, increasing the likelihood of detecting transferred DNA.
Next comes mixtures. Crime scene samples are rarely clean single-source DNA; they usually contain material from multiple contributors. When labs analyze these mixtures, they must make assumptions about how many contributors are present and which markers belong to whom. Misinterpretation of mixtures can easily implicate someone who never touched the object or was never present. The presence of an individual’s DNA in a mixture does not provide context about how or when it was deposited, yet jurors often interpret it as definitive evidence.
Software interpretation further complicates matters. Programs like STRMix are used to analyze complex mixtures through probabilistic genotyping, producing likelihood ratios that describe how likely it is that a particular person could be included as a contributor. While mathematically sophisticated, these results are conditional on the assumptions input by analysts—such as the number of contributors, peak thresholds, and DNA drop-in or drop-out probabilities. The software produces probabilities, not certainty, but in court these numbers are often presented as near-absolute proof of involvement.
The final link in the chain is misstated statistics. Prosecutors, intentionally or not, can overstate what a likelihood ratio or “cannot be excluded” statement actually means. Phrases like “this DNA matches the defendant” or “there is only a one in a million chance someone else could have left this DNA” are easy for jurors to hear as conclusive. In reality, these numbers are conditional and do not reflect the probability that the defendant committed the crime. This misunderstanding is known as the prosecutor’s fallacy and is a frequent driver of wrongful convictions.
When these factors combine, the result can be devastating. Consider a realistic scenario: an innocent person works in a shared office. A co-worker who later commits a theft handles a piece of office equipment and accidentally transfers the innocent person’s DNA to it. The stolen item is recovered, tested, and analyzed as a mixture. STRMix calculates a likelihood ratio strongly favoring the inclusion of the innocent person’s DNA. The prosecutor presents the evidence to the jury with confident language. Even though the person never touched the item during the crime, the jury interprets the DNA presence as proof of guilt, leading to conviction.
These situations are not hypothetical. The Innocence Project and other organizations have documented cases where individuals were wrongly convicted primarily because their DNA appeared at a crime scene without any other corroborating evidence. Often, the DNA found was from trace or touch material, came from a mixture, or was interpreted through probabilistic software, all combined with overstated statistics. In many cases, these convictions were later overturned after deeper scientific review or post-conviction testing clarified the context.
The lesson is clear: a DNA “match” is not synonymous with guilt. Modern testing is extraordinarily sensitive and mathematically complex, but sensitivity and sophistication do not automatically translate to courtroom certainty. When DNA transfer, mixtures, software interpretation, and statistical misrepresentation are layered together, the result can be convincing but misleading. Defense attorneys must carefully challenge these assumptions and clarify what the evidence truly shows.
Understanding how innocent people end up convicted on DNA “matches” is essential not just for legal professionals, but for anyone interpreting forensic evidence. DNA is a powerful tool, but it is not infallible, and relying on it without context or scrutiny can have life-altering consequences. Recognizing the limitations of DNA evidence helps ensure that probabilistic, transferred, or mixed DNA is treated as one piece of the puzzle—not the whole story.
In forensic DNA reporting, precise language is essential because the science is inherently probabilistic, not absolute. Laboratories rarely use the term “match” in their reports; instead, they describe results with phrases such as “cannot be excluded,” “consistent with,” or by providing a likelihood ratio. These expressions convey uncertainty and account for assumptions about contributors, mixture complexity, and the quality of the sample. The careful scientific language reflects that DNA evidence demonstrates presence of genetic material, not proof of who left it, when it was deposited, or how it got there. In the courtroom, however, this nuance is often lost. When experts summarize their findings, or prosecutors simplify the testimony for jurors, the measured terms from the lab can be translated into the shorthand of a “DNA match,” which is far more definitive in everyday understanding than the science actually allows.
The problem with the word “match” is that it carries a misleading implication of certainty. In common usage, a match suggests exact equivalence—two puzzle pieces fitting perfectly or fingerprints definitively linking to a single person. When jurors hear that a defendant’s DNA “matched” evidence, they often assume the lab has proven the individual was at the crime scene or handled an object in question. In reality, phrases like “cannot be excluded” indicate only that the person’s profile could be included as a contributor under specific assumptions; it does not establish timing, method, or direct involvement. This disparity between DNA match vs inclusion means jurors may give the evidence far more weight than it deserves. Avoiding the shorthand of “match” and emphasizing the actual language of DNA reports helps ensure that juries understand what the evidence truly shows: genetic material consistent with a person, not definitive proof of guilt.
Challenging DNA evidence is a critical component of effective criminal defense, particularly when a case relies heavily on probabilistic results or complex mixtures. One of the first strategies is challenging the assumptions underlying the analysis. Analysts must make numerous decisions when interpreting DNA: the number of contributors to a mixture, which genetic peaks are meaningful, and how to treat low-level or ambiguous signals. A defense attorney can scrutinize these assumptions, highlighting how small changes can dramatically alter the statistical outcomes. By questioning the reliability of the initial assumptions, attorneys demonstrate that DNA results are not infallible and must be considered alongside other evidence.
Another common strategy is challenging the interpretation of mixtures. Most crime scene samples contain DNA from multiple individuals, creating overlapping peaks and ambiguous data. Defense attorneys may call attention to how analysts separate these contributors, emphasizing the subjectivity involved in deciding which signals belong to which individual. Even advanced software cannot eliminate all uncertainty. By showing that mixtures require interpretation—and that different analysts might reach different conclusions—the defense can reduce the perceived certainty of the DNA evidence in the eyes of a jury.
Modern forensic labs frequently rely on probabilistic genotyping software, such as STRMix, to handle complex mixtures. While this software is mathematically sophisticated, its results depend on user inputs and assumptions. Defense attorneys can question the reliability of the software itself, pointing out the “black box” nature of proprietary programs. Cross-examination may focus on how the program works, whether it has been independently validated, and whether its results have been challenged or interpreted differently in other cases. By doing so, the defense underscores that software-generated conclusions are not infallible and are subject to limitations.
Cross-examining analysts is another critical tactic. Attorneys often ask detailed questions about training, protocols, and potential sources of error in the lab. For instance, analysts may be asked to explain how they handled low-level DNA, how they determined contributor numbers, or how they addressed ambiguous peaks. By pressing on these points, the defense can reveal the human judgment inherent in the testing process and the potential for mistakes or differing interpretations. This helps jurors understand that DNA results are not absolute proof of involvement.
Defense attorneys may also pursue pretrial motions under Rule 702 and Rule 403 to limit or exclude certain DNA evidence. Rule 702 addresses the admissibility of expert testimony, requiring that methods be scientifically valid and properly applied. Rule 403 allows courts to exclude evidence if its potential to mislead the jury outweighs its probative value. Attorneys might argue that complex probabilistic results, overstated statistics, or evidence of trace DNA could confuse jurors, creating a risk that the evidence will be given undue weight. Even if the motion is not granted, raising these issues educates the judge about the limitations of the evidence.
Ultimately, challenging DNA evidence is about context, clarity, and critical scrutiny. Defense attorneys aim to show that DNA is powerful but not infallible, and that every result depends on human assumptions, interpretation of mixtures, and software calculations. By questioning assumptions, examining mixtures, evaluating software reliability, cross-examining analysts, and invoking appropriate legal standards, attorneys ensure that jurors view DNA evidence as one piece of the puzzle rather than definitive proof of guilt. In this way, a robust defense transforms the mystique of DNA into a carefully analyzed, transparent part of the trial.
Related reading: Challenging DNA evidence in court: defense tactics that matter.
DNA evidence can seem infallible in the courtroom, but jurors should approach it with careful scrutiny. Understanding the limits and context of DNA testing is critical to evaluating its probative value. One of the first questions jurors should ask is: “Is this a mixture?” Most crime scene samples are not clean, single-source DNA. Objects such as clothing, tools, or doorknobs often carry genetic material from multiple people. Knowing whether the evidence is a mixture helps jurors understand that interpretation is more complex than simply comparing one profile to a suspect—it may involve multiple contributors and statistical modeling rather than direct observation.
Jurors should also ask: “How many contributors are there?” Determining the number of people who contributed to a DNA sample is not straightforward. Analysts make educated assumptions based on the number of genetic peaks and their patterns. These assumptions can significantly impact the statistical results and the strength of the evidence linking a suspect to the scene. Understanding how many contributors the lab assumed—and whether other interpretations are possible—helps jurors assess the reliability of the conclusions.
Another essential question is: “What assumptions were made?” Every step of DNA analysis, especially in mixtures, relies on assumptions. Analysts decide how to treat faint or ambiguous peaks, how to handle potential contamination, and which parameters to enter into software like STRMix. Even small variations in these assumptions can produce very different likelihood ratios or inclusion probabilities. By asking about these assumptions, jurors gain insight into the degree of judgment involved and the limitations of the conclusions being presented.
Perhaps the most important question is: “Is this evidence showing presence, or proof of action?”DNA can show that someone’s genetic material is on an object or at a location, but it cannot show when it was deposited, how it got there, or whether a crime occurred. Jurors should be mindful that detection of DNA does not automatically imply the person touched the item during the crime, committed an act, or had intent. Understanding this distinction prevents the common misconception that a DNA “match” equals guilt.
By asking these critical questions about DNA evidence, jurors are empowered to evaluate forensic testimony thoughtfully and accurately. These inquiries encourage jurors to look beyond simplified courtroom language like “match” or “cannot be excluded” and to focus on the scientific reality: DNA evidence is powerful, but it is probabilistic, conditional, and always subject to interpretation. A careful approach ensures that DNA is considered as one piece of the broader case rather than treated as unassailable proof.
Related reading: 10 questions to ask your criminal defense attorney if DNA evidence is being used in your case.
DNA evidence is one of the most powerful tools in modern criminal investigations, but its strength can also be its greatest source of misunderstanding. Popular culture, courtroom shorthand, and media portrayals have created the impression that a “DNA match” is absolute proof of guilt. In reality, DNA does not tell a story of what happened, when it happened, or who was involved. A “match” typically means only that your genetic material could be included as a contributor under certain assumptions—assumptions about the number of contributors to a mixture, how peaks are interpreted, or how probabilistic software calculates likelihood ratios. Without understanding these nuances, DNA evidence can appear far more definitive than it truly is.
It is important to recognize that a DNA match is not a direct link to criminal behavior. It does not indicate when DNA was deposited, how it got to a particular location, or whether any crime actually occurred. Touch DNA, secondary transfer, and complex mixtures can all lead to the appearance of a match even when the individual had no involvement in the alleged act. Probabilistic genotyping software, like STRMix, adds another layer of complexity, producing statistical likelihood ratios that are conditional on human assumptions and interpretation. Misrepresenting these results as absolute proof is a common source of misunderstanding in trials and can contribute to wrongful convictions.
Because of these complexities, proper legal scrutiny is essential. Defense attorneys play a vital role in challenging assumptions, analyzing mixtures, questioning software reliability, and ensuring that statistical results are accurately communicated to jurors. They help jurors distinguish between DNA presence and proof of action, ensuring that evidence is considered in context rather than taken as an unassailable fact. Without careful review, juries may overvalue DNA evidence, giving it far more weight than it scientifically deserves.
If you are facing criminal charges involving DNA evidence, it is critical to have a lawyer who understands both the science and the law. DNA can be compelling, but without expert guidance, it can also be misleading. Contact Ginny to ensure that your case is thoroughly evaluated, your rights are protected, and the nuances of DNA evidence are fully understood.
Understanding the real meaning of a DNA match is not just a matter of science—it is a matter of justice. DNA is powerful, but its true value comes from context, critical analysis, and expert legal guidance. Recognizing that a match does not equal guilt can make the difference between conviction and justice.
If DNA evidence (including touch DNA, mixtures, or STRMix likelihood ratios) is being used against you in Minnesota, the details matter.
Learn more about attorney Ginny Barron and how she approaches forensic-driven defense cases.
When you’re ready, you can contact us here to request a confidential consultation.
No. A “DNA match” does not prove guilt. It means a person’s genetic profile is consistent with DNA detected in a sample, often under a set of assumptions made by a lab or software. DNA shows presence of genetic material, not how, when, or why it got there. It cannot tell a jury whether someone committed a crime, only that their DNA could be included as a possible contributor to a sample.
Yes. DNA can be transferred through secondary transfer. For example, a handshake, shared clothing, or common surfaces can move trace DNA from one person to another object without direct contact. This is especially true with modern testing methods that can detect extremely small amounts of genetic material, sometimes called “trace” or “touch” DNA.
Touch DNA (also called trace DNA) refers to tiny amounts of skin cells left behind when a person handles or comes near an object. These samples often contain very little material and are easily affected by transfer, contamination, and environmental factors. Touch DNA does not prove a person handled an object at the time of a crime—only that genetic material was found there.
A DNA mixture is a sample that contains genetic material from two or more people. Most crime scene samples are mixtures. Interpreting these mixtures is complex and often requires assumptions about how many contributors are present and which genetic markers belong to whom. These interpretations are not direct observations; they are statistical estimates.
STRMix is a type of probabilistic genotyping software used by crime labs to interpret complex DNA mixtures. Instead of a human analyst deciding which DNA markers belong to which person, the software uses algorithms to calculate likelihood ratios. This result is not a “match,” but a statistical opinion about whether a person could be included as a contributor under certain assumptions.
Yes. DNA evidence can be misunderstood, overstated, contaminated, or misinterpreted—especially in cases involving touch DNA, mixtures, and probabilistic software. Errors do not mean the science is useless, but they do mean DNA results must be carefully examined in the context of how the sample was collected, tested, and explained in court.
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