How Reliable Is DNA Evidence? A Criminal Defense Attorney Breaks It Down

May 13, 2025

When it comes to criminal cases, DNA evidence is often portrayed as a silver bullet. But is it really that conclusive? In this post, Attorney Brittany Paz explores the complex realities of DNA testing, the reliability of forensic reports, and the critical role of Porter hearings in Connecticut courts. Here are the top insights from her latest video on what every defense attorney—and client—should know.

1. DNA Reports Have Changed—And Not Always for the Better

From Alleles to Probabilities

In the past, DNA reports presented data using allele charts to compare genetic material. Today, these reports are dominated by probability language. Rather than a straightforward "yes" or "no" regarding whether someone’s DNA is present, reports now use phrases like “more likely than others in the population.” This shift introduces a layer of uncertainty that requires a critical eye.

2. Complex Mixtures: The Hidden Risk in DNA Evidence

What Are Complex Mixtures?

When multiple individuals handle an object, it often results in a complex DNA mixture. Forensic scientists must estimate how many contributors were involved—2, 3, 4, or even more.

The Problem with Probability Modeling

For instance, if there's:

40% likelihood of three contributors

20% likelihood of four

10% likelihood of more than four

Analysts default to the highest percentage even if it's well below 50%. This creates a potential for unreliable conclusions to be introduced at trial.

3. When Is DNA Too Complex to Be Reliable?

The Case of the Two Guns

Attorney Paz discusses a case involving two firearms:

Gun #1 had more than four DNA contributors, making the sample too complex to analyze.

Gun #2 had a mixture just under that threshold. Still, the report was based on assumptions regarding how many contributors were involved.

In both situations, the results raise serious doubts about admissibility and reliability.

4. Challenging DNA Evidence: The Role of a Porter Hearing

What Is a Porter Hearing?

In Connecticut, the legal process for questioning the scientific validity of evidence is called a Porter hearing (similar to a Daubert hearing in federal court). During this hearing:

Forensic lab personnel testify about their methods.

The court determines if the science behind the testing is reliable enough to present at trial.

If the evidence doesn't meet the threshold, it may be excluded.

5. Lessons from Polygraph Tests: A Cautionary Tale

Polygraphs were once widely used but are no longer admissible in trials due to their questionable reliability—something established through Daubert hearings. The same fate could await complex DNA mixtures if courts find the assumptions and probabilities scientifically unreliable

6. The Stakes Are High: DNA Evidence Can Make or Break a Case

If DNA evidence is excluded after a Porter hearing, prosecutors may lose their key piece of evidence, potentially influencing their decision to pursue charges at all. This legal grey area makes DNA analysis a critical battleground in criminal defense.

DNA Evidence Isn’t Always What It Seems

Attorney Brittany Paz emphasizes that DNA evidence isn’t always as reliable as it seems—especially when based on complex mixtures and probability models rather than clear genetic comparisons. These assumptions can lead to questionable evidence being presented in court. At Pattis & Paz, we know how to challenge unreliable forensic methods and protect your rights. If you're facing charges involving DNA or forensic evidence, don’t leave your defense to chance. Contact us for a free consultation and let our experienced New Haven legal team fight for you.