DNA evidence tends to carry dramatic weight in a courtroom. Jurors hear phrases like “one in a billion” and often assume complete scientific certainty. But is DNA analysis truly a perfect science? How can DNA evidence be misunderstood or misapplied?
The Science of DNA Evidence
Deoxyribonucleic acid, also known as DNA, is the unique genetic blueprint for the cells of the human body. Except in cases of identical twins, no two individuals share the same DNA.
The entire human genome contains nearly 20,000 protein-coding genes, as well as a plethora of non-coding genes. For that reason, forensic investigators do not sequence or analyze an entire genome. They focus their analysis on small repeating regions known as Short Tandem Repeats (STRs). These regions of DNA vary significantly between individuals, making them highly useful for identification.
In the United States, forensic labs examine a standardized set of 20 core STR loci used in the national database system known as CODIS. Each person has two alleles at each locus, one inherited from each parent. The combination of all tested loci forms a DNA profile.
DNA Analysis in Criminal Cases
Here is a simple outline of what the DNA Analysis process may look like:
1. Collection – Biological specimen such as blood, semen, saliva, or skin cells is collected from a scene.
2. Extraction – DNA is isolated from other cell materials.
3. Quantification – The amount of DNA available is measured.
4. Amplification – Using Polymerase Chain Reaction (PCR), selected DNA regions are copied millions of times.
5. Profile Generation – The lab measures DNA fragment sizes and generates a DNA profile.
6. Comparison – This profile is compared to a known suspect’s reference sample, or to profiles stored in CODIS when no suspect is known.
Benefits of DNA Evidence
- The Power of Exoneration – DNA evidence has been used to exonerate many wrongfully convicted individuals in recent years
- Standardized Process – Rules and Regulations mandated by the government help ensure the process follows a standard protocol
- Extensive Research and Testing – DNA Analysis has been studied in depth over the last few decades through peer-reviewed research
Limitations and Risks
- Sample Contamination – Poor collection and lab practices can lead to contamination of collected specimens
- Mishandling and Laboratory Error – Mistakes in the lab such as improper labeling can lead to false results
- Contextual Misunderstanding – Juries often are misinformed about DNA evidence and treat it as proof of guilt when it’s presented in court
- Mixed Samples – Collected samples can contain more than one individual’s DNA
The Complication of Mixed DNA
Real-world DNA samples are often mixtures of multiple individuals. For instance, a sexual assault victim may have had consensual sex earlier the same day. A vaginal swab could contain DNA from two male contributors, as well as the victim’s.
Labs detect mixtures when more than two alleles appear at a single locus. Investigators must then:
- Estimate the number of contributors
- Separate profiles if feasible
- Apply statistical analysis
The Efficacy of DNA Analysis
DNA analysis cannot determine when it was deposited, how it was deposited, or whether contact was consensual. It only shows that biological material consistent with a person’s DNA is present.
The science of DNA itself is robust and highly accurate. The human application of it is where risk of error appears. Misuse or misunderstanding can create wrongful convictions, which is why this evidence must be properly collected, processed, interpreted, and explained to a jury.
DNA analysis is one of the most scientifically backed tools in modern forensic criminal investigation. However, its persuasive power in court can overshadow public understanding of its limitations. As with all forensic evidence, the key is to avoid blind acceptance.