DNA Feels Like Certainty. The Report Is an Interpretation.
A DNA result looks like a simple yes or no. It is not. It is the end of a long chain of human decisions, from how the sample was collected to how an analyst read the data and which statistical model they chose to run. Each step can introduce error, and each step can be tested.
I began my career as an Assistant Public Defender in Florida’s Thirteenth Judicial Circuit, and I am one of only six attorneys in Florida, and just over a hundred in the country, recognized as a Forensic Lawyer-Scientist by the American Chemical Society. On the breath and blood chemistry I am trained in, I read the raw data myself. On DNA, fingerprints, cell-site, digital, and medical evidence, I bring in the right expert and cross-examine the State’s analyst on method, on chain of custody, and on the assumptions hiding inside the conclusion. Learn more about my background.
Where DNA Cases Go Wrong
The common problem areas include mixtures of more than one person’s DNA, which are open to subjective interpretation; low-level or touch samples, where there is barely enough material to read reliably; transfer and secondary transfer, where your DNA ends up somewhere you never went because it rode in on an object or another person; contamination during collection or inside the lab; and a CODIS database cold hit, which is an investigative lead and not proof that you did anything.
Even the statistics are contestable. Two laboratories running different probabilistic genotyping programs can report meaningfully different numbers from the very same mixture, which tells you the match is not the hard fact it appears to be.
Reading the Data, Not the Conclusion
I have an independent analyst review the electropherograms, the bench notes, and the raw electronic data, not just the one-page summary. Under Florida’s Daubert standard in section 90.702, we can challenge whether the method was reliable and whether it was applied correctly to this sample.
A Match Is Not the Same as Proof
When an analyst reports a match, what they are really reporting is a statistic: how rare this profile is in the population. That number can be impressive, but it answers a narrow question. It does not say how the DNA got there, when it was deposited, or whether it has anything to do with the alleged crime. A person can leave DNA on an object that later travels to a scene they never visited, and the statistic says nothing about that.
Partial profiles make the picture murkier still. When only some of the genetic markers are readable, the rarity statistic weakens, the chance of coincidental similarity rises, and the room for interpretation grows. The State’s report rarely dwells on any of this, which is why an independent read of the data matters so much.
Touch DNA, Contamination, and Lab Error
Touch or trace DNA, left by brief contact, is now collected routinely, but it is exactly the kind of low-level sample most prone to misreading and to transfer. A few cells are easy to move from place to place and easy to misinterpret.
Laboratories are run by people, and people make mistakes: samples get switched, controls fail, and contamination happens, sometimes from the lab’s own staff or equipment. Proficiency testing and accreditation help, but they do not erase the risk, and reviewing the lab’s records can reveal problems that never appear in the final report.
How I Defend a DNA Case
I get the full laboratory file early, bring in the right geneticist, and look hard at collection, storage, interpretation, and the database hit. The goal is to show the jury the difference between a number on a page and proof beyond a reasonable doubt.
Getting the Right Records Early
Most of what wins a DNA challenge lives in records the State does not hand over with the report. That includes the raw electronic data files the instrument produced, the analyst’s bench notes, the laboratory’s validation studies for the software it used, and its history of proficiency testing and any past errors. We ask for all of it early, in writing, because once we have the underlying data, an independent geneticist can rerun the interpretation and see whether the State’s conclusion was the only reasonable one or just the one the analyst chose.
Common Questions
Can DNA evidence be wrong?
Yes. DNA depends on how a sample was collected, stored, and interpreted. Mixed and low-level samples are especially open to error, and an independent analyst reviewing the raw data can find problems the State’s report never mentions.
What is a DNA mixture and why does it matter?
A mixture is DNA from more than one person in a single sample. Pulling one person’s profile out of a mixture involves subjective judgment, and different analysts and different software can reach different conclusions, which is exactly where the defense lives.
What is secondary transfer?
It is how your DNA can end up on something you never touched, by riding in on another person or a shared object. It means a DNA match does not prove you were ever at a scene.
Does a CODIS hit prove it was me?
No. A cold hit from the DNA database is a lead for investigators, not proof of guilt. The underlying analysis still has to hold up, and it often does not survive a close look.
This page is general information about Florida law, not legal advice, and it does not create an attorney-client relationship. Florida applies the Daubert standard for expert testimony under section 90.702, Florida Statutes. The reliability of forensic evidence depends on the methods, the analysts, and the facts of each case, and the science and the law can change, so confirm anything here with counsel about your own situation. Every case is different, and past results do not guarantee a similar outcome.
