The paradigm shift in DNA profile interpretation

DNA mixtures occur when two or more individuals contribute to a sample. Mixtures can vary greatly in complexity. Key variables are the number of donors, the template amount of each donor’s DNA, and the level of DNA degradation of each donor. There is little published material that we can find about behaviours relating to the limits of DNA mixture interpretation in the 90 s and 00s. Fromour own personal knowledge, we recall that two-person mixtures were regularly examined in the 90 s and that a ratio of 10:1 was considered a reasonable limit beyond which the profile wasconsidered too complex to interpret. Three-person and higher order mixtures were seldom attempted.
It is difficult to find a clear statement of the logic for the 10:1 rule of thumb. Clayton and Buckleton ([1] citing Clayton et al. [2]) state that the alleles of the minor must be above background noise: “the threshold represents approximately a 1:10 mixture ratio . . . ". This would suggest that the 10:1 rule of thumb is consequential on the analytical threshold (AT) and not causal. For a modern 3500 capillary electrophoresis (CE) machine the AT is often of the order of 50–200 rfu. Peaks can be up to 30,000 rfu in height. This suggests that a major contributor’s peak heights around 20,000 rfu and a trace contributor’s peak heights around 100 rfu would both be in
the analysable range. This is a ratio of 200:1.
In Section 2 we will explore the risk of interpreting high ratio mixtures using a probabilistic genotyping software. Section 3 discusses recent issues related to validation, including the PCAST report, four-person mixtures, the effect of replication and known contributors, and the magnitude of validation tests. The discussion and conclusion follow in Sections 4 and 5, respectively.