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DNA Mixtures:
A Forensic Science Explainer

Rich Press

What are DNA Mixtures? And why are they sometimes so difficult to interpret?

Everyone's DNA is slightly different. Thirty-five years ago, when scientists invented a way to distill those differences into a DNA profile, they revolutionized forensic science. That profile—a sort of genetic fingerprint—gave investigators a new and extremely reliable tool for solving crimes.

DNA profiling can be so powerful and has been used successfully in so many cases that some people think it is nearly infallible. But the reliability of DNA profiling varies, and other things being equal, it is most reliable when the evidence contains plenty of DNA from just one or two people. That's why homicides and sexual assaults—crimes that tend to produce a lot of this type of DNA evidence—made up the bulk of DNA casework for many years.

But in the last decade or so, forensic experts have been analyzing DNA mixtures, which occur when the evidence contains a mixture of DNA from several people. They are also analyzing trace amounts of DNA, including the "touch DNA" left behind when someone touches an object. These types of evidence can be far more difficult to interpret reliably than the DNA evidence typical of earlier decades. With old-school DNA, the results tend to be clear cut: either a suspect's DNA profile is found in the evidence or it isn't, and nonexperts can readily understand what that means. With DNA mixtures and trace DNA, the results can be ambiguous and difficult to understand, sometimes even for the experts.

These more complex types of evidence make up an increasing share of the DNA casework in the United States, and labs are rapidly adopting new methods and tools to deal with them. More than just an upgrade to the existing tool set, these changes represent a fundamentally new approach to DNA evidence and mark a profound shift in the field. Given the great weight that DNA evidence carries in the courtroom, it is important that lab analysts, criminal investigators, judges, attorneys—and anyone who might sit on a jury someday—understand these changes. So here's a quick primer on DNA mixtures and trace DNA, what makes them difficult to interpret, and what these changes mean for the future of the field.

Why have DNA mixtures and trace DNA become so prevalent?

DNA methods have become extremely sensitive. Forensic scientists once needed a relatively large amount of material, such as a visible blood or semen stain, to produce a DNA profile. Today, they can generate a profile from just a few skin cells that someone left behind when touching an object or surface.

This capability is an incredible technological achievement. It is also a positive development because it allows forensic science to help solve a greater variety of crimes. Investigators might be able to solve a sexual assault, for instance, even if very little DNA were recovered. They might investigate a break-in by swabbing the pry bar that was used to force a door, or they might swab a firearm that was used to commit a crime.

But such high sensitivity is a double-edged sword. We often shed small amounts of DNA when we talk, sneeze and touch things. As a result, many surfaces are likely to contain mixtures of minute amounts of DNA from several people. These mixtures have always been present at crime scenes, but when sensitivity was lower, they wouldn't have been detected or, if they were, labs would not have attempted to interpret them. That is no longer the case.

Are all DNA mixtures difficult to interpret?

Some mixtures are relatively easy to interpret. Others are more complex and require greater care. Still others may be too complex to reliably interpret at all. It depends on the specifics of the case.

Three main factors determine the complexity of a mixture.

  • How many people contributed DNA to the mixture? More contributors make a mixture more complex, and therefore, more difficult to interpret.

  • How much DNA did each person contribute? Even if a mixture contains plenty of DNA overall, one or several people might have contributed only a tiny amount. The lower those amounts, the more complex the mixture.

  • How degraded is the DNA? DNA degrades over time and with exposure to the elements. This can also increase complexity.

  • When does a DNA mixture become too complex to reliably interpret at all? Currently, there are no established standards for deciding this. Different labs have different protocols. When confronted with a particularly complex DNA mixture, some labs will try to interpret it and others won't.

< read the complete article >

Article published by the National Institute of Standards and Technology.

Related Content
  • National Forensic DNA Study Report
  • Report to the Attorney General on Delays in Forensic DNA Analysis
  • Using DNA to Solve Cold Cases: Special Report

  • How can NGS Solve More Cases?

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