What are STRmix™ and Probabilistic Genotyping?

When an evidence DNA profile comes from a single person, it’s easy to interpret. Add extra people to a profile, and the profile becomes more difficult especially when there are three or four people involved as contributors. Further complications arise when there may be very little DNA from one of the contributors.

Probabilistic genotyping combines statistical approaches with computational power to resolve DNA mixtures that would not be possible to analyze by human thinking alone. One probabilistic genotyping software is STRmix™, and Sorenson Forensics currently uses the software to analyze complex DNA profiles.

  • It’s faster than more manual methods.
  • DNA analysts can easily explain and defend STRmix™ in court.
  • Calculate a likelihood ratio (LR) against person of interest profiles. This compares the probabilities of prosecution and defense scenarios to explain the evidence.
  • Under challenging circumstances, such as when a DNA sample is very small or contains a mixture of the DNA of several people, it may be utilized in place of manual techniques.


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Why is Probabilistic Genotyping Necessary?

Traditional DNA interpretation involves human analysis of profiles to determine if two profiles are consistent or if a person could have been a contributor to a profile. When a DNA profile is relatively simple such as from a single person, the profiles can be easy to interpret since it’s straightforward to compare signals between evidence profiles and profiles from persons of interest.

Mixture DNA is more difficult to interpret. Multiple people may contribute similar DNA markers to an evidence profile, and the people may have contributed relatively different amounts of DNA. Depending on case circumstances, a human analyst can straightforwardly interpret two-person mixtures. When mixtures get to three- and four-person, it becomes progressively more difficult to interpret. The overlap of DNA markers and the multitude of possible combinations to explain profiles make it difficult for a human to interpret.

A traditional approach to mixture interpretation is combined probability of inclusion (CPI) which looks at the probability of people who could contribute to a profile. It does not consider the separate individual profiles that could combine to produce the evidence profile. Probabilistic genotyping considers the individual profiles that could contribute and thus makes better use of the genetic information.

Let’s talk and determine if probabilistic genotyping can assist you with your case.

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