How Is DNA Evidence Evaluated in a Texas Murder Case?
A DNA result can point to a possible contributor. It usually can’t say how the DNA got there, when it arrived, or what happened next.
Mr. Heath Enix 'Heath' Hyde
Attorney at Law · Eligible to Practice in Texas
Bar Card Number: 00796807
TX License Date: 11/01/1996
Primary Practice Location: Sulphur Springs, TX and Dallas, TX
Office Address: 214 Connally St Ste A
Sulphur Spgs, TX 75482-2612
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Short answer
A DNA match tells you whose DNA might be on an item. It doesn’t tell you how it got there, when, or what happened next. So the defense skips the one-page summary and asks for the lab’s whole file.
Listen for the State’s shorthand in court.
| What the State may say | What the result may show |
|---|---|
| “His DNA was on the knife.” | One part of a mixture on the handle. |
| “One in a billion.” | A likelihood ratio, not the odds of guilt. |
| “Her DNA proves she was there.” | DNA can move without direct contact. |
| “The lab confirmed it.” | An analyst made choices the defense can review. |
What questions can a DNA result answer?
A DNA result answers one question well and two others poorly. NIST’s 2024 foundation review treats the “who” and “how” questions as separate levels of interpretation.
- Whose DNA? The lab’s profile comparison usually addresses whether a person could be a contributor.
- How did it get there? Other facts in the case have to answer this one, not the profile alone.
- When was it left? Contributor results alone don’t establish whether the DNA arrived during the offense or at another time.
Key point: A strong answer to “whose DNA” can leave the other two questions wide open.
What makes a DNA mixture harder to read?
A mixture holds DNA from more than one person. Say a swab from a kitchen knife picks up the owner, a guest from last weekend, and someone the lab can’t name. That’s a three-person mixture before anyone asks about the crime.
Some features make a mixture harder to read:
- Each added contributor makes the mixture more complex to separate.
- A tiny contribution from one person is harder to read reliably.
- Time and exposure to the elements can break DNA down.
- NIST notes there’s no established standard for when a mixture becomes too complex to interpret reliably.
Labs often run complex mixtures through probabilistic genotyping software. The analyst still chooses the inputs, so the defense asks about each one:
- How many contributors did the analyst assume? A different number can change the result.
- Which software, version, and settings did the lab use?
- Did the lab validate the software on samples like this one?
- Did the analysis assume any relatives or known contributors?
What does a likelihood ratio mean?
Lab reports often give a number like “1 million times more likely if the defendant and two unknown people contributed than if three unknown people did.” That’s a likelihood ratio. It compares two explanations for the same DNA data, and it isn’t the chance of guilt.
Two assumptions sit behind that number. The first is whether an unknown contributor is “unrelated.” If so, the math assumes no close relative, so a brother or a cousin isn’t ruled out. The second is how many people contributed. A four-person mixture can look like a three-person one, and the software builds everything on the analyst’s count.
The Federal Judicial Center’s guide for judges makes the same point: a likelihood ratio isn’t a probability of guilt or innocence.
How could DNA get on an item without the person touching it?
DNA moves. It rides on hands, sleeves, and evidence bags, and it can land on an item hours after the crime. Here are four ways it can show up where nobody put it. The first is called secondary transfer.
| Scenario | The question it raises |
|---|---|
| Roommates shake hands; one touches a doorknob. | The other’s DNA can reach the knob. |
| A deputy bags a shirt and a knife together. | DNA may have moved between the items. |
| The lab swabs evidence near a reference sample. | Ask for the contamination log. |
| The defendant lived where police found the item. | Everyday contact may explain the DNA. |
What to do now: Ask for the evidence-collection log, the lab’s contamination log, and its negative controls. Those records show how each item was packaged and handled.
How does the defense test the analyst?
Texas law gives the defense specific tools.
| Rule | What it lets the defense do |
|---|---|
| TRE 702 | Challenge qualifications and whether the opinion helps the jury. |
| TRE 705(b), (c) | Question the expert about the data outside the jury’s hearing, and object if the basis is insufficient. |
| CCP art. 38.35(d) | Exclude analysis from a lab that wasn’t accredited then, with limited exceptions. |
Article 39.14 also gets you the State’s reports and favorable information. If you ask in writing by the 30th day before jury selection, the State must name its experts by the 20th day before.
The Confrontation Clause matters, too, when the analyst who ran the test doesn’t testify.
| Case | What the Court held |
|---|---|
| Melendez-Diaz v. Massachusetts (2009) | Sworn forensic lab certificates were testimonial, so the analysts were witnesses the accused could confront. |
| Smith v. Arizona (2024) | A substitute expert’s relayed statements that support the opinion only if true come in for their truth. |
Can the defense get its own DNA testing?
Often, yes. What the law allows depends on where the case is:
- Before trial, CCP art. 38.43 requires the agencies that hold biological evidence in a felony case to retain and preserve it for set periods.
- In a capital case where the State seeks death, art. 38.43(i) requires DNA testing of biological evidence before trial, and (m) lets the defense use another accredited lab for more testing.
- After a conviction, CCP chapter 64 lets a convicted person ask for DNA testing when identity was or is an issue and other conditions are met.
- If the science changes after trial, CCP art. 11.073 allows habeas relief based on new scientific evidence in certain cases.
What can a family do while the DNA work is pending?
Think about who was in the house that month. Maybe a cousin slept on the couch for a week. Maybe a neighbor borrowed the knife set for a cookout. Facts like those never show up in a lab report, but they can explain a mixture.
Make a short list for the defense lawyer:
- Everyone who lived in or visited the home in the two weeks before, with rough dates
- Who used the item day to day
- Shared laundry, towels, or cars
- Texts, receipts, or photos that place your loved one somewhere else
Hand the list to the lawyer, not to the police or the lab.
Key point: Jail phone calls are usually recorded. Don’t discuss the DNA results on those calls or online.
Frequently asked questions
Does a DNA match prove my son committed the murder?
No. A DNA result speaks to whose DNA might be present, not how or when it got there or who caused the death.
What if the DNA belongs to someone else?
It may point to another person who was there. Your lawyer should get the full data, including any unidentified profiles.
Can a bad DNA lab result be challenged after trial?
Sometimes. Chapter 64 and art. 11.073 set narrow conditions, so talk with counsel about them early.
Should our family hire a DNA expert?
That’s a decision for the defense lawyer. An independent expert can review the lab file, the software run, and the statistics.
Talk with Heath Hyde
If DNA is part of the case against someone you love, the full lab file matters more than the summary. Call Heath Hyde, P.C. at 903.439.0000 or request a free consultation. You can also read what the State must prove in a Texas murder case and the forensic evidence section of the Texas Murder Trial Defense Knowledge Hub.
Legal sources
- Tex. Code Crim. Proc. arts. 38.35, 38.43
- Tex. Code Crim. Proc. art. 39.14
- Tex. Code Crim. Proc. ch. 64
- Tex. Code Crim. Proc. art. 11.073
- Tex. R. Evid. 702–705
- Smith v. Arizona, 602 U.S. 779 (2024)
- Melendez-Diaz v. Massachusetts, 557 U.S. 305 (2009)
- NIST IR 8351, DNA Mixture Interpretation (2024)
- NIST, DNA Mixtures: A Forensic Science Explainer
- Federal Judicial Center, Probabilistic Genotyping Systems
This article provides general educational information, not legal advice. Every case depends on its facts, evidence, court, deadlines and governing law. Past results do not guarantee a similar outcome.
