Probabilistic Genotyping
Definition
Software-based interpretation of complex DNA electropherograms using continuous statistical models (e.g. STRmix, TrueAllele, ArmedXpert). Generates a likelihood ratio for complex mixtures previously classed as uninterpretable. Sensitive to software version, parameter choices, and lab-specific settings.
- Core tools
- STRmix, TrueAllele, EuroForMix, ArmedXpert
- What it models
- Stochastic PCR effects, allelic drop-out, allelic drop-in
- Key output
- Likelihood ratio for profile attribution
Common questions
What's the main difference between probabilistic genotyping and regular DNA profile analysis?+
Regular analysis relies on fixed binary allele calls (allele present or absent), while probabilistic genotyping uses continuous statistical models to account for stochastic effects in PCR, drop-out, and drop-in. This lets analysts work with complex mixtures that traditional binary methods would classify as uninterpretable.
What does the likelihood ratio tell you in probabilistic genotyping?+
The likelihood ratio compares the probability of the DNA evidence given two competing hypotheses, like whether a suspect contributed to the mixture. Higher ratios support the hypothesis of contribution. The software computes this by modeling allelic drop-out and drop-in rather than forcing yes/no calls on every allele.
Are all probabilistic genotyping software tools the same?+
No. Results can be sensitive to the software version (STRmix, TrueAllele, EuroForMix, ArmedXpert), parameter choices, and lab-specific settings. This means two labs might interpret the same profile slightly differently depending on their tools and configurations.
Related terms
- Allelic Drop-Out
- Failure of one allele of a heterozygous locus to amplify detectably, a stochastic effect of low-template amplification where sampling of limited template...
- ENFSI Verbal Scale
- A standardised table published by the European Network of Forensic Science Institutes that maps ranges of LR values to verbal expressions of...
- ESSplex SE QS
- Qiagen kit covering the European Standard Set of 17 loci plus DYS391 and Amelogenin; optimised for EU Prüm database upload.
- Evaluative Reporting
- A framework for expressing forensic conclusions in terms of the probability of the evidence given the prosecution hypothesis versus the defence hypothesis...
- Familial Searching
- The deliberate use of partial DNA profile matches in a forensic database to identify biological relatives of an unknown contributor, on the...
- Forensic Database
- A searchable repository of DNA profiles, fingerprints, images, or other forensic data from known individuals, crime scenes, or both. Includes national DNA...
- GlobalFiler
- Applied Biosystems 25-plex STR kit covering 24 STR loci plus Amelogenin, compatible with both CODIS 20 and ESS17 requirements.
- Haematin
- Degradation product of haemoglobin that inhibits Taq polymerase by chelating Mg2+ ions and binding to the polymerase active site; most common inhibitor...
- Humic Acid
- Organic polymer from soil degradation that inhibits PCR by intercalating into double-stranded DNA and disrupting primer annealing.
- Identifiler Plus
- Applied Biosystems 16-plex STR kit covering the original 13 CODIS loci plus D2S1338, D19S433, and Amelogenin. Widely used through the 2010s; insufficient...
- Investigator 24Plex QS
- Qiagen 26-plex STR kit with Quality Sensor (IPC) and covering 24 STR loci plus DYS391 and Amelogenin. GO! variant uses increased cycles...
- IPC (Internal PCR Control)
- A non-forensic DNA sequence spiked into the PCR reaction at fixed concentration; a shift in IPC Ct value relative to the expected...
Explained in these topics
- Interpreting Biological Evidence in CourtA computational approach to interpreting DNA mixtures that models all possible genotype combinations across contributors, stutter, drop-in, and drop-out, produ...
- Multiplex STR Kits, Inhibitor Handling and Low-Template DNAComputational framework (STRmix, TrueAllele, EuroForMix) that models stochastic PCR effects and computes a likelihood ratio for profile attribution, accounting...
- The Future of Forensic Science: AI, Databases, Rapid MethodsSoftware-based interpretation of complex DNA electropherograms using continuous statistical models (e.g. STRmix, TrueAllele, ArmedXpert). Generates a likelihoo...