How Your DNA—or Someone Else’s—Can Send You to Jail

Although DNA is individual to you—a “fingerprint” of your genetic code—DNA samples don’t always tell a complete story. The DNA samples used in criminal prosecutions are generally of low quality, making them particularly complicated to analyze. They are not very concentrated, not very complete, or are a mixture of multiple individual’s DNA—and often, all of these conditions are true. If a DNA sample is like a fingerprint, analyzing mixed DNA samples in criminal prosecutions can often be like attempting to isolate a single person’s print from a doorknob of a public building after hundreds of people have touched it. Despite the challenges in analyzing these DNA samples, prosecutors frequently introduce those analyses in trials, using tools that have not been reviewed and jargon that can mislead the jury—giving a false sense of scientific certainty to a very uncertain process. This is why it is essential that any DNA analysis tool’s source code is made available for evaluation. It is critical to determine whether the software is reliable enough to be used in the legal system, and what weight its results should be given. 

A Breakdown of DNA Data

To understand why DNA software analyses can be so misleading, it helps to know a tiny bit about how it works. To start, DNA sequences are commonly called genes. A more generic way to refer to a specific location in the gene sequence is a “locus” (plural “loci”). The variants of a given gene or of the DNA found at a particular locus are called “alleles.” To oversimplify, if a gene is like a highway, the numbered exits are loci, and alleles are the specific towns at each exit.

[P]rosecutors frequently introduce those analyses in trials, using tools that have not been reviewed and jargon that can mislead the jury—giving a false sense of scientific certainty to a very uncertain process.

Forensic DNA analysis typically focuses on around 13 to 20 loci and the allele present at each locus, making up a person’s DNA profile. By looking at a sufficient number of loci, whose alleles are distributed among the population, a kind of fingerprint can be established. Put another way, knowing the specific towns and exits a driver drove past can also help you figure out which highway they drove on.

To figure out the alleles present in a DNA sample, a scientist chops the DNA into different alleles, then uses an electric charge to draw it through a gel in a method called electrophoresis. Different alleles will travel at different rates, and the scientist can measure how far each one traveled and look up which allele corresponds to that length. The DNA is also stained with a dye, so that the more of it there is, the darker that blob will be on the gel.

Analysts infer what alleles are present based on how far they traveled through the gel, and deduce what amounts are present based on how dark the band is—which can work well in an untainted, high quality sample. Generally, the higher the concentration of cells from an individual and the less contaminated the sample by any other person’s DNA, the more accurate and reliable the generated DNA profile.

The Difficulty of Analyzing DNA Mixtures

Our DNA is found in all of our cells. The more cells that we shed, the higher the concentration of our DNA can be found, which generally also means more accuracy from DNA testing. However, our DNA can also be transferred from one object to another. So it’s possible that your DNA can be found on items you’ve never had contact with or at locations you’ve never been. For example, if you’re sitting in a doctor’s waiting room and scratch your face, your DNA may be found on the magazines on a table next to you that you never flipped through. Your DNA left on a jacket you lent a friend can transfer onto items they brush by or at locations they travel to. 

Given the ease at which DNA is deposited, it is no surprise that DNA samples from crime scenes are often a mixture of DNA from multiple individuals, or “donors.” Investigators gather DNA samples by swiping a cotton swab at the location that the perpetrator may have deposited their DNA, such as a firearm, a container of contraband, or the body of a victim. In many cases where the perpetrator’s bodily fluids are not involved, the DNA sample may only contain a small amount of the perpetrator’s DNA, which could be less than a few cells, and is likely to also contain the DNA of others. This makes trying to identify whether a person’s DNA is found in a complex DNA mixture a very difficult problem. It’s like having to figure out whether someone drove on a specific interstate when all you have is an incomplete and possibly inaccurate list of towns and exits they passed, all of which could have been from any one of the roads they used. You don’t know the number of roads they drove on, and can only guess at which towns and exits were connec