DNA evidence has transformed criminal justice over the past three decades. The ability to link a person to a crime scene or victim through biological evidence seems like objective science. However, DNA interpretation—particularly in complex cases involving mixed samples, degraded DNA, or trace amounts—is far more nuanced than popular media suggests.
The Evolution of DNA Interpretation
When DNA analysis first became available in the late 1980s and early 1990s, the focus was straightforward: inclusion or exclusion. Either the DNA profile from a crime scene matched the suspect, or it didn’t. A match meant the person was likely at the crime scene; no match meant they could be excluded.
This simple model works well for single-source DNA profiles—situations where the crime scene sample contains DNA from only one person. But in reality, many crime scenes yield mixed samples containing DNA from multiple people: the victim, the suspect, and possibly other innocent individuals present at the location.
Over the past 15 years, DNA interpretation has evolved significantly. Modern experts use activity-level propositions rather than source-level conclusions. Instead of simply saying “the DNA matches the suspect,” experts now address the underlying question: “What activity explains the presence of this person’s DNA at this location?”
Understanding Source Level vs. Activity Level
Source-level analysis asks: “Could this DNA have come from this person?” It provides a straightforward match/no-match determination. This is useful for excluding suspects but limited for complex mixtures.
Activity-level analysis asks: “What activity or behavior would explain the presence of this person’s DNA at the crime scene?” This is far more informative but also requires more careful analysis.
Example of the difference: Imagine a murder case where the victim was killed in her home, and DNA analysis finds a mixture of the victim’s DNA and the suspect’s DNA under the victim’s fingernails. At the source level, the conclusion is simply that the suspect’s DNA is present in the sample. At the activity level, that same finding is consistent with a struggle where the victim scratched the assailant—but it could also be explained by innocent activities such as shaking hands, prior consensual contact, or transfer from a shared environment.
Activity-level interpretation requires the expert to consider the biological transfer mechanisms at play, the likelihood of DNA transfer under different scenarios, and the probability of an innocent explanation versus guilt.
Mixture Analysis and the Challenge of Multiple Contributors
Mixed DNA samples create significant challenges. When multiple people’s DNA is present in a single sample, determining the individual profiles requires sophisticated analysis. Low-level mixtures (DNA from two to three people in obvious proportions) can usually be resolved into component profiles. Complex mixtures (multiple people in unclear proportions, or with degradation) create interpretation challenges. Modern software like STRmix and TrueAllele uses probabilistic genotyping to estimate the most likely individual profiles, but these estimates include uncertainty ranges.
Implications for defense: If the prosecution presents DNA evidence from a complex mixture and concludes it matches the defendant, a sophisticated defense strategy should question the mixture assumptions (how many contributors are assumed?), challenge the software output (what assumptions underlie the likelihood ratio?), and hire a counter-expert to run the same data—potentially reaching different conclusions.
Probabilistic Genotyping: The New Standard
Traditional DNA analysis assigned profiles to individuals based on simple rules: if a peak is present at a locus, the person contributes that allele. With degraded DNA or complex mixtures, these rules break down. Probabilistic genotyping uses Bayesian statistics to estimate the probability of different profile combinations given the observed data. Instead of declaring “this person’s profile is X,” it calculates the probability of that profile given what was observed.
This approach is more powerful for complex mixtures and degraded samples, but it also introduces uncertainty ranges (no more simple “match” conclusions), hypothesis-dependence (the probability depends on the prosecution versus defense scenarios), and software-dependence (different software can produce different answers for the same data).
Key Questions for Defense Attorneys
- Is the sample single-source or mixed? Single-source evidence is far more straightforward.
- Is the expert’s conclusion source level or activity level? Activity-level conclusions appropriately include more uncertainty.
- If mixture analysis was used, what assumptions were made, and how sensitive are the results to different assumptions?
- If probabilistic genotyping was used, what is the likelihood ratio, and does it support conviction or reasonable doubt?
- Has the analysis been reviewed by a qualified counter-expert?
- Does the DNA evidence alone meet the burden of proof, or does the case rely on other evidence?
DNA Is Important but Not Infallible
DNA evidence has revolutionized criminal justice, but modern interpretation is far more complex than the popular image of DNA analysis. If you’re facing charges where DNA evidence is involved, representation by an attorney with genuine forensic DNA expertise is critical. Jarrett Ambeau brings both trial advocacy and advanced forensic science knowledge to challenge questionable DNA evidence and protect your rights.