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‘DNA Mixtures,’ ‘Touch DNA,’ and Software-Enhanced Forensic DNA Analysis

by Michael Berk

The premise that a DNA “match” conclusively overrides nearly all other evidence in a criminal case has become a deep-seated one over the 35 years since the inception of DNA profiling. Often, though, such views are not founded upon even a basic understanding of how this powerful forensic tool is used to turn the most microscopic materials into some of the most reliable evidence.

Less DNA Can Mean More Evidence

In the early days of DNA profiling, only substantial quantities of blood, semen or hair could suffice to produce a useful evidentiary result. But just about every cell in (or, as will be noted, on) the human body contains DNA, and over time, the science has evolved to take advantage of this fact. Today, not only is it possible to run tests on very small samples, but the practice of using software to sift “irrelevant” information from the materials under analysis, while retaining enough fidelity to hold up in court, has expanded DNA profiling far beyond its early boundaries.

One of the most impactful advances in the field has been experts’ ability to work with “trace” amounts of genetic material. And because testing methods have grown so sensitive as to allow for analysis of minute amounts of DNA, forensic experts have been able to make increasing use of what is known as “touch DNA.”

Throughout the course of a day, our skin is constantly renewing itself—and as old skin cells are replaced by newer layers, these cells (and the DNA they contain) are left behind when any of us touch a doorknob, use a keypad, or lean against a wall. The barest smattering of such cells can be developed into robust evidence of an individual’s physical presence, if conditions are suitable.

That “if,” however, is a big one, and as usual, the devil is in the details. What does it mean to say that a given person’s DNA has been identified at a specific location or on a particular object?

Your DNA Can Go Places You’ve Never Been

One big issue with this new technique is known as “secondary transfer.” As already mentioned, people shed cells all the time, including just by coughing, sneezing—even talking. We leave identifiable bits of ourselves everywhere we go; yet, counterintuitively, our DNA ends up in places we’ve never been, as well.

For example, if you grip a door handle to enter a building right before someone else does the same, and that person goes on to use the same hand to touch things at the scene of a crime, the DNA you left on the doorknob may well end up as forensic evidence because of “secondary transfer”: the skin cells you left behind, in this example, were picked up from the door and transferred to the second person’s hand, who then left your DNA somewhere you had never been.

For that matter, a sample collected for DNA analysis may contain genetic material from more than one person. In the past, when large quantities of such material were necessary to produce a forensic report, the smaller “blips” in the data caused by phenomena like secondary transfer wouldn’t have been very noticeable—or at least were so plainly distinguishable, by size, from the bulk of the evidence as to be easily and confidently excluded from the results as nuisance interference.

But now that smaller amounts of DNA can form a complete analytic profile, to the extent that a given sample does contain material from multiple sources, it is imperative that forensic experts be able to “separate out” each contributor’s DNA “signature” from such “complex DNA mixtures.”

Whose DNA Is It Anyway?

One fact that makes this possible is that labs only concern themselves with about 40 short segments of human DNA. That is because these 40 “alleles” are the most tellingly variant from person to person. Although people always have some alleles in common, it’s extremely unlikely that two different people will share all 40 of these “genetic markers.”

Unfortunately, the way DNA samples are presently analyzed means that just because a particular set of alleles is present in the final results does not demonstrate that a given person having all of those alleles contributed to the sample: There could have been two (or more) people, each having some of the indicated alleles, who supplied material for the sample. (Imagine two contributors, each having alleles 1, 2, 3, 4 and 5, 6, 7, 8, respectively—the results would include the set 3, 4, 5, 6 ... but that doesn’t mean that someone having the distinct makeup 3, 4, 5, 6 contributed to the sample.)

That’s why experts use probabilistic genotyping software (“PGS”) to assist in the interpretation of complex DNA mixtures. These programs use statistical and biological models to determine the probability that the information revealed by testing evidences the presence of a known person’s DNA.

The software accounts for aberrations commonly encountered in DNA analysis, and comes up with a “likelihood ratio.” This number takes into consideration the frequency with which each allele (in partnership with other alleles) appears in the general population, and is an estimate of how likely it is that the particular set of alleles arising from the test of the sample came from the specified human subject.

Developing Evidence From DNA

The application of PGS may be the point in DNA forensics at which it is most vulnerable. In many instances, PGS assists in improving the reliability of interpretations of complex DNA mixtures—but only to the extent that the analyst is fully informed as to the assumptions and mathematics involved.

On the other hand, the fine-tuning inherent in experts’ use of PGS can dramatically affect the results of DNA analyses. Based on the use of different settings and other factors controlled by the person operating the software, different labs might produce different answers from the same sample. Those differences can sometimes call into question the reproducibility of the test results. It seems that, as with many high-precision tools, the finer the resolution of the question asked, the more uncertain the answer returned.

This caveat remains especially serious when dealing with “trace” amounts of DNA. Due to the extremely high value placed on DNA forensics, it is important to remember that just because a person’s DNA may be associated with a particular object or location does not necessarily mean that the person had anything to do with it.

The responsibility to effectively interpret DNA evidence, as any evidence, resides with the finders of fact in each case. Perhaps now more than ever before, thinking critically about the meaning of DNA forensic reports, in conjunction with other available evidence, and thoroughly understanding the methods, limitations, and factors inherent in DNA profiling, are crucial components of ensuring sound outcomes in criminal justice. 



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