# DNA Profile Match Probability Calculator
This DNA profile match probability lab helps students and curious readers explore how a small autosomal STR profile becomes a probability under a simplified population genetics model. Enter two alleles and their frequencies for each locus. The calculator applies the Hardy Weinberg genotype rule and multiplies the locus frequencies to show a classroom random match probability.The interface keeps the calculation visible: the trace shows the entered alleles, the table shows each genotype frequency, and the final result is expressed as one in a power of ten. The frequencies in the presets are synthetic teaching values, not an official population database. # How the Genotype Rule Works
| Two copies of one allele | p² | The allele frequency is multiplied by itself. |
| Two different alleles | 2pq | The two allele frequencies are multiplied and doubled. |
| Several independent loci | Product of locus values | Each locus contributes to the combined profile frequency. |
# Why Population Frequencies Matter
Allele frequencies are estimates from population databases. The relevant population, sampling method, database size, substructure, relatedness, and validation all affect how confidently a genotype frequency can be used. A synthetic preset can demonstrate the arithmetic, but it cannot stand in for a validated forensic database.The result is best understood as a conditional statement: if the entered frequencies describe the relevant population, if the profile is a clean single source, and if the loci are treated as independent, then the product estimates how often that profile could appear in unrelated random people. Changing any of those assumptions can change the interpretation. The tool intentionally keeps the data editable so that this dependency is visible rather than hidden behind a single authoritative looking number.In a real case, analysts also document how a profile was selected, whether a database search influenced the investigation, whether the sample is a mixture, and whether relatives or population substructure need special treatment. Those questions are outside this small laboratory. Use the trace and table to explain the arithmetic, then use the limitations to explain why arithmetic alone is not a forensic opinion. -
Use one population context: Do not mix frequencies from unrelated databases.
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Keep assumptions visible: This lab assumes unrelated contributors and simplified independence between loci.
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Separate probability from identity: A rare profile is not a statement about guilt, source, or legal certainty.
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Document the data: A formal report needs the source, population, method, uncertainty, and interpretation.
# Educational Limits
Use this as a model, not a verdict
This calculator does not analyse electropherograms, model mixtures, correct for population substructure, apply theta corrections, estimate likelihood ratios, or search a DNA database. It is for learning how simple genotype frequencies combine.