The Gold Standard
DNA profiling revolutionized forensic science when Alec Jeffreys developed the technique in 1984 and first applied it to a criminal case in 1986 — simultaneously exonerating an innocent suspect and identifying the true perpetrator. Today, DNA evidence is the most powerful identification tool available to law enforcement, with random match probabilities that can exceed one in a sextillion when full profiles are obtained.
How STR Profiling Works
Modern forensic DNA typing analyzes Short Tandem Repeat (STR) loci — specific chromosomal locations where a short sequence (e.g., AGAT) repeats a variable number of times. Each person inherits two copies (alleles) per locus, one from each parent. By typing 20 loci (the current CODIS standard), analysts generate a profile that is statistically unique. PCR amplification allows profiling from as little as 100 picograms of DNA — about 15 cells.
The Product Rule
The strength of DNA evidence comes from the product rule: the probability of a multi-locus match is the product of the individual locus probabilities. If each locus has a genotype frequency of 1-5%, multiplying across 13 or 20 loci yields combined probabilities of 10⁻¹⁵ to 10⁻³⁰. This multiplication is valid when loci are on different chromosomes (genetically independent), which is true for all CODIS loci.
Challenges and Mixtures
Not all DNA evidence is straightforward. Crime scenes often yield mixed profiles from multiple contributors, degraded samples with allele dropout, or touch DNA with very low template amounts. Probabilistic genotyping software uses likelihood ratios to evaluate mixed and partial profiles statistically. Sample degradation, modeled in this simulation, reduces the number of usable loci and weakens the statistical power of the match.