Empirical Analysis of the STR Profiles Resulting from Conceptual Mixtures

Abstract

Samples containing DNA from two or more individuals can be difficult to interpret. Even ascertaining the number of contributors can be challenging and associated uncertainties can have dramatic effects on the interpretation of testing results. Using an FBI genotypes dataset, containing complete genotype information from the 13 Combined DNA Index System (CODIS) loci for 959 individuals, all possible mixtures of three individuals were exhaustively and empirically computed. Allele sharing between pairs of individuals in the original dataset, a randomized dataset and datasets of generated cousins and siblings was evaluated as were the number of loci that were necessary to reliably deduce the number of contributors present in simulated mixtures of four or less contributors. The relatively small number of alleles detectable at most CODIS loci and the fact that some alleles are likely to be shared between individuals within a population can make the maximum number of different alleles observed at any tested loci an unreliable indicator of the maximum number of contributors to a mixed DNA sample. This analysis does not use other data available from the electropherograms (such as peak height or peak area) to estimate the number of contributors to each mixture. As a result, the study represents a worst case analysis of mixture characterization. Within this dataset, approximately 3% of three-person mixtures would be mischaracterized as two-person mixtures and more than 70% of four-person mixtures would be mischaracterized as two-or three-person mixtures using only the maximum number of alleles observed at any tested locus.

Magnitude-dependent variation in peak height balance at heterozygous STR loci

“…Not all mixtures of two or more individuals exhibit three or more peaks at every locus, particularly when information is only available at a small number of loci such as often occurs with samples containing small amounts of template DNA and/or those that are degraded and/or inhibited [13][14][15][16][17][18][19][20][21]. In such circumstances, a significant peak height imbalance at a locus where only two alleles are observed may still be indicative of a mixed sample [15][16][17][18][19][20][21].…”

“…In such circumstances, a significant peak height imbalance at a locus where only two alleles are observed may still be indicative of a mixed sample [15][16][17][18][19][20][21]. Peak height information is less reliable with low quantities of DNA template mainly due to modifications to the testing process and PCR sampling issues.…”

Does an English appeal court ruling increase the risks of miscarriages of justice when complex DNA profiles are searched against the national DNA database?

“…The question of 'matching alleles' in multi-contributor DNA profiles has been previously characterised for complete profiles [15,16]. It was previously shown [17] that the (common) practice of calculating an RMNE from loci where the suspect's alleles were fully included, whilst omitting those loci where alleles were not fully represented, was bad practice, and anti-conservative.…”

The variability in likelihood ratios due to different mechanisms

“…A 2005 study [5] showed that it can also be difficult to determine how many people have contributed to a mixture. The authors showed that more than 70% of four person mixtures could be incorrectly designated as two or three person mixtures.…”