DNA Evidence — The Most Reliable Forensic Science in Criminal Cases
DNA is the most reliable forensic evidence — but it can still be challenged. Phoenix defense attorney Matt Long explains how, and what every Arizona defendant should know.
"When you really have a full sample of DNA, we know who that is. DNA is a known thing. Once you get beyond that, hair, bite marks, amylase, body language, you are in the world of junk science, and everything changes."
What Makes DNA the Gold Standard of Forensic Evidence
DNA evidence meets the criteria that most forensic methods fail: it is observable, measurable, testable, replicable, and based on fixed biological identity that can be objectively verified. A complete, properly collected, and uncontaminated DNA profile can connect a specific individual to a specific biological sample with a statistical certainty that no other forensic tool approaches.
This is why DNA evidence has exonerated over 375 wrongfully convicted people in the United States, including people on death row. When DNA was properly analyzed and the results honestly applied, innocence was established beyond any reasonable doubt. No other forensic method has that track record of producing reliable results in both directions: proving guilt when someone is guilty, and establishing innocence when someone is not.
- Based on fixed biological identity
- Objectively verifiable and replicable
- Known statistical error rates
- Has exonerated wrongful convictions
- Can establish both guilt and innocence
- Internationally standardized methodology
- Subjective examiner judgment
- Poor or unknown error rates
- Rarely peer reviewed rigorously
- Have contributed to wrongful convictions
- Cannot establish innocence reliably
- Methodology varies by examiner
Four Ways a Defense Attorney Can Challenge DNA Evidence
Accepting DNA evidence without scrutiny is a mistake, even when the profile matches your client. The reliability of DNA evidence depends entirely on how the sample was collected, handled, stored, and analyzed. Errors at any point in that chain can produce results that appear definitive but are not.
Contamination
DNA samples can be contaminated at the scene, during collection, in transport, or in the laboratory. A contaminated sample may produce a false match, identifying a person whose DNA was introduced into the evidence through an error in the process, not through contact with the crime.
Transfer
DNA transfers from person to object to person without direct contact between individuals. Finding someone's DNA at a crime scene proves only that their genetic material is present, not that they were there at the time of the crime. Secondary and tertiary transfer are well-documented phenomena.
Partial Profiles
When a DNA sample is degraded, mixed, or insufficient, laboratories produce partial profiles with fewer genetic markers. Partial profiles have higher rates of coincidental matches and lower statistical certainty. Prosecutors routinely present partial profiles with the same confidence as complete ones.
Chain of Custody
Any gap or irregularity in the documented record of who collected, handled, and analyzed a sample creates reasonable doubt about the integrity of the evidence. A defense attorney examines chain of custody records as a first step, not an afterthought.
The same investigative failures that corrupt physical evidence can corrupt witness accounts, a pattern examined in detail in our analysis of how investigators contaminate evidence through improper handling.
Not All DNA Profiles Are Equal: Mixed and Partial Samples
A DNA match is only as strong as the profile behind it. Two categories of weaker profile appear constantly in criminal cases, and prosecutors often present both with a confidence the underlying data does not support.
Mixed Profiles
A mixed profile occurs when biological material from more than one person is present in a single sample. Mixtures are far harder to interpret than single-source samples. They carry higher rates of analytical error, and the statistical calculations applied to them can mislead a jury about how certain a match really is. When two or more contributors are present, separating one person's genetic markers from another is an interpretive act, not a mechanical readout. That interpretation is open to challenge, and an independent expert can test whether the laboratory's conclusion is the only one the data allows.
Partial Profiles
A partial profile results when a sample is degraded, contaminated, or too small to produce a complete analysis across all tested markers. Fewer markers mean a higher chance of a coincidental match to someone who had nothing to do with the crime. A partial profile is not a complete profile presented with an asterisk. It is a different and weaker form of evidence. Arizona Rule of Evidence 702 requires that its limitations be established before it reaches a jury. Prosecutors sometimes blur that line. A defense attorney draws it back.
Rule 702: The Gate Every Forensic Method Must Pass
Forensic evidence does not reach an Arizona jury simply because the State wants to use it. Arizona Rule of Evidence 702 sets the standard. The methodology must be testable, it must have a known or knowable error rate, it must have survived peer review, and it must be generally accepted in the relevant scientific community. Arizona applies the Daubert framework, which puts the trial judge in the role of gatekeeper.
DNA analysis satisfies all four criteria. That is what separates it from the methods that have sent innocent people to prison. Bite mark analysis, microscopic hair comparison without DNA confirmation, and body language interpretation fail on multiple criteria. The distinction is not a matter of opinion. It is a legal standard, and a defense attorney enforces it through pretrial motions that test whether the State's proposed evidence belongs in front of a jury at all.
According to the Innocence Project, flawed or misapplied forensic science contributed to roughly 45 percent of wrongful convictions later overturned by DNA. That figure includes both junk science and legitimate methods that were overstated or presented without their limitations. Rule 702 exists to keep that evidence out before a jury ever hears it. Litigating the gate is often where a case is won or lost.
DNA exonerated Ray Krone after Arizona put him on death row based on bite mark analysis. That is what real forensic science does: it tells the truth even when institutions do not want to hear it.Matt Long, Partner, Long & Simmons Law
When Real Chemistry Produces False Conclusions
One of the most dangerous categories of forensic error is not junk science but misapplied science, where a legitimate scientific concept is used to draw a conclusion it cannot support. The amylase example is instructive.
Amylase is a real enzyme found in saliva, and its chemistry is well understood. Prosecutors have used amylase swabs from a victim's body to argue that a specific person committed a sex act against that person. The problem is that amylase is also present in other bodily fluids, can be transferred through clothing or environmental contact, and cannot be traced to a specific individual without a DNA analysis. The science of amylase is real. The conclusion drawn from finding it is not.
Prosecutors frequently present DNA evidence alongside weaker forensic methods, including bite mark analysis, hair matching, and amylase swabs, in a single case. The effect on a jury is cumulative: each piece of evidence appears to reinforce the others, making the overall case seem stronger than any individual piece would justify. A defense attorney must challenge each method independently and clearly explain to the jury why the weight of evidence is not the same as the reliability of evidence.
The Ray Krone Precedent in Arizona
Ray Krone was convicted of murder and sentenced to death in Arizona based on bite mark analysis, a method since thoroughly discredited, while DNA evidence that could have exonerated him was not properly developed. When that DNA was eventually analyzed, it identified another individual as the actual perpetrator. Krone was released after spending more than a decade in prison, including time on death row.
His case represents the best and worst of forensic science in Arizona criminal law: the worst, because junk science sent an innocent man to death row; the best, because real science, DNA, ultimately told the truth. The lesson is not that forensic science is uniformly unreliable. It is that only some forensic science meets the standard that justice demands, and defense attorneys must know the difference.
How DNA Is Collected in Sex Crime Cases
In sex crime cases, DNA is typically collected through a Sexual Assault Nurse Examiner exam, known as a SANE exam. A specially trained nurse conducts a forensic medical examination, usually within 72 to 120 hours of a reported assault, and takes swabs from areas of reported contact. Those samples go to a crime laboratory for analysis.
Two distinct points of failure exist in that process, and both are open to challenge. The first is collection: whether the examiner followed established protocol, documented the chain properly, and avoided contamination. The second is laboratory analysis: whether the testing was performed correctly and the results interpreted within the limits of the sample. A defense attorney examines both. A deviation at either stage can compromise the reliability of a result that the State presents as conclusive.
How a Defense Attorney Evaluates and Challenges DNA Evidence
DNA Evidence in Phoenix and Maricopa County Courts
Forensic evidence in Maricopa County criminal cases is often processed through state and local crime laboratories, and the quality of that work varies. Laboratory accreditation status, internal quality control audits, and proficiency testing histories are all discoverable by the defense. Those records can reveal contamination problems, backlogs, or processing errors at a specific facility, and they are among the first things a defense attorney requests.
The Ray Krone case is not distant history in this state. Arizona convicted Krone of murder and sentenced him to death on bite mark evidence while the DNA that would clear him sat undeveloped. When that DNA was finally analyzed, it identified the actual perpetrator and Krone walked free after more than a decade, including time on death row. The case is a permanent reminder of what is at stake when forensic evidence is taken at face value in an Arizona courtroom.
If you are facing DNA or other forensic evidence in a Phoenix or Maricopa County criminal matter, the methodology behind that evidence can be examined, tested, and challenged. Long & Simmons Law handles criminal defense throughout Maricopa County and across Arizona, from pre-file investigation through trial.
DNA challenges and investigative failures often arise in the same case, especially when reviewing how incomplete investigations produce false narratives.
Matt Long — Long & Simmons Law
Matt Long
A criminal defense attorney and nationally recognized expert in child and intimacy crimes, Matt Long provides training to law enforcement agencies and professionals across the country on best practices in investigations and trial presentations. A former prosecutor in sex crimes, child crimes, and computer crimes, he brings over twenty years of experience to Arizona's most sensitive criminal cases, first for the state, now seeking justice, protecting the rights of his clients, and holding law enforcement accountable.
Facing DNA Evidence in an Arizona Criminal Case?
Even the most reliable evidence can be challenged. Call Long & Simmons Law for a confidential consultation with an attorney who understands forensic science at the trial level.
Questions about dna & forensic evidence
Is DNA evidence always reliable in criminal cases?
DNA evidence from a complete, properly collected, and uncontaminated sample is the most reliable forensic evidence available. However, even DNA evidence can be challenged on grounds of contamination, secondary transfer, partial profiles, chain of custody errors, and laboratory processing mistakes. Over 375 wrongful convictions in the United States have been overturned using DNA — demonstrating that it reveals truth in both directions, proving guilt and establishing innocence with equal reliability when handled correctly.
What is DNA transfer and how does it affect a criminal case?
DNA transfer occurs when genetic material moves from one person or object to another without direct contact — through handshakes, shared surfaces, clothing, or secondary transfer via a third party. Finding someone's DNA at a crime scene does not prove they were present at the time of the crime. A defense attorney investigates all plausible innocent transfer mechanisms before conceding that DNA evidence establishes presence or contact at the relevant time.
What is a mixed DNA profile and how does it affect a criminal case in Arizona?
A mixed DNA profile occurs when biological material from more than one person is present in a single sample. Mixed profiles are significantly more difficult to interpret than single-source profiles, have higher rates of analytical error, and can produce misleading statistical calculations. Prosecutors have presented mixed DNA profiles in Arizona courts in ways that overstate the certainty of a match. Defense attorneys retain independent DNA experts to evaluate the mixture interpretation methodology and challenge conclusions that the underlying data does not support.
What is a partial DNA profile and why does it matter?
A partial DNA profile results when a sample is degraded, contaminated, or too small to produce a complete genetic analysis across all tested markers. Partial profiles have higher rates of coincidental matches than full profiles and are considered significantly less reliable. Under Arizona Rule 702 of the Rules of Evidence, the reliability of the methodology — including the limitations of partial profiles — must be established before such evidence reaches a jury. Prosecutors sometimes present partial profiles with the same stated confidence as complete ones, and defense attorneys expose that difference.
How does chain of custody affect DNA evidence in Arizona courts?
Chain of custody is the documented record of every person who collected, handled, transferred, and analyzed a piece of evidence from the crime scene to the courtroom. Any gap, irregularity, or undocumented transfer in that chain creates a reasonable question about whether the sample that was tested is the same sample that was collected. Arizona defense attorneys examine chain of custody records as the first step in evaluating DNA evidence, and file motions to suppress when the chain cannot be established.
Can DNA evidence be contaminated during collection or laboratory analysis?
Yes. DNA evidence can be contaminated at the crime scene during collection, during transport, during storage, and in the laboratory during processing. Contamination can occur through direct contact with an investigator's DNA, through environmental exposure, or through cross-contamination between samples in the laboratory. Laboratory accreditation records, internal quality control audits, and proficiency testing histories are all discoverable by the defense and can reveal systemic contamination problems at a specific facility.
What percentage of wrongful convictions involved flawed forensic evidence?
According to the Innocence Project, flawed or misleading forensic science contributed to approximately 45 percent of wrongful convictions later overturned by DNA evidence. This includes not only junk science like bite mark analysis and microscopic hair comparison, but also legitimate forensic methods that were misapplied, overstated, or presented to juries without adequate explanation of their limitations.
How is DNA collected in sex crime cases in Arizona?
In sex crime cases in Arizona, DNA is typically collected through a Sexual Assault Nurse Examiner (SANE) examination — a forensic medical exam conducted by a specially trained nurse within 72 to 120 hours of a reported assault. Swabs are taken from areas of reported contact. The samples are then submitted to a crime laboratory for analysis. Defense attorneys examine both the collection protocol and the laboratory analysis to identify any deviation from established procedures that could compromise the reliability of the results.
What is the difference between forensic evidence and junk science in criminal cases?
Forensic evidence meets the scientific standard required by Arizona Rule of Evidence 702: the methodology must be testable, have a known error rate, have been subjected to peer review, and be generally accepted in the relevant scientific community. Arizona applies the Daubert framework, placing the trial judge in the role of gatekeeper. DNA analysis meets all four criteria. Junk science — including bite mark analysis, hair follicle matching without DNA, and body language interpretation — fails on multiple criteria and has contributed to documented wrongful convictions. The distinction is not a matter of opinion; it is a legal standard that defense attorneys enforce through pretrial motions.
How does a defense attorney challenge DNA evidence in Arizona courts?
An Arizona defense attorney challenging DNA evidence will: obtain and review the raw electropherogram data rather than just the laboratory report summary; examine the laboratory's accreditation status and proficiency testing history; investigate all plausible DNA transfer mechanisms; challenge the statistical presentation of match probabilities for partial or mixed profiles; and retain an independent DNA expert to provide an alternative interpretation of the evidence where the underlying data supports one. Each step is grounded in the Arizona Rules of Evidence and the scientific literature on forensic DNA analysis.
General educational purposes only. Not legal advice. Contact a licensed Arizona attorney immediately if you are under investigation or have been charged.