The Mixture Challenge
When DNA from multiple individuals is combined in a single sample, the resulting STR profile becomes a superposition of contributor genotypes. At each locus, allele peaks overlap — two contributors can produce 2 to 4 distinct peaks per locus, three contributors up to 6, and so on. The challenge lies in determining how many people contributed, what their individual genotypes are, and how strongly the evidence supports or excludes a person of interest.
Peak Heights and Contributor Ratios
Peak heights in Relative Fluorescence Units (RFU) are roughly proportional to the amount of DNA template. In a two-person mixture, if one person contributed three times as much DNA, their peaks will be approximately three times taller. By examining peak height patterns across loci, analysts can estimate the mixture ratio and assign alleles to major and minor contributors. When the ratio exceeds 10:1, the minor contributor's alleles may be indistinguishable from analytical noise.
Stochastic Effects and Dropout
At low template amounts, PCR amplification becomes stochastic — one allele of a heterozygote may fail to amplify sufficiently, a phenomenon called allele dropout. The stochastic threshold defines the peak height below which dropout cannot be excluded. For minor contributors in heavily skewed mixtures, dropout rates can exceed 30%, making manual interpretation unreliable and necessitating probabilistic approaches.
Probabilistic Genotyping
Modern forensic laboratories increasingly use probabilistic genotyping software that considers all possible genotype combinations, weights them by population allele frequencies, and models peak height variance and stochastic effects. These systems output likelihood ratios comparing the prosecution hypothesis (person of interest is a contributor) against the defense hypothesis (an unknown random person contributed instead), providing quantitative weight to mixture evidence.