DNA by the numbers

Nowadays increasing numbers of evidentiary traces are collected at crime scenes and submitted for DNA analysis at the forensic laboratories. However, almost 50% of the analyzed DNA samples do not result in valuable DNA typing information (1) and a few studies show that the possibility to actually obtain usable DNA profiles can depend on the trace type (2,3). Evaluating the DNA results obtained for various sampled traces can provide us information on which traces are most promising to select for DNA analysis. Such information can guide crime scene investigators in decision-making.

The study

Six European forensic laboratories1 from the EUROFRGEN network, gathered DNA yields from over 24,466 crime-related samples that were categorized based on biological source or sampled item.  The category ‘sample type’ includes various biological sources such as bodily fluids and tissues and the category ‘sampled item’ includes several items sampled for either saliva or contact traces.

DNA yield was used to predict the DNA profiling result.  Four categories were chosen based on in-house experience: 1) full profile, 2) usable partial or full profile, 3) partial profile possibly useful, and 4) no informative profile. Details on this categorization can be found in Table 1. These four categories inform us which are the most promising samples to select for DNA analysis.

Observations and conclusions

A total of 44 categories were made for the overall categories ‘sample type’ and ‘sampled item’. The number of samples in each category varies from 18 to 7104 and the results represent trends. In Figure 1 for each sample category, the percentages of samples with an expected type of profile are shown: dark and middle green bars indicate full and usable profiles; a light bar represents possibly useful profiles and a brown bar marks the category no profile. Within the overall categories, the sample categories are ranked from lowest to highest percentage no profile expected.

When comparing sample types, we see for instance that for blood samples in 93% of the cases a full profile and in 4% no profiles may be obtained. For feaces samples, on the other hand, the percentage no profile is much higher namely 24%. This variation is also observed when comparing various sampled items likely to carry saliva or contact traces: the percentage in the ‘no profile’ category is 2% for balaclavas and 29% for bottle lids and 0% for coat collars and 44% for plastic bags.

The proximity, intensity and duration of contact seem to contribute to profiling success as saliva items balaclava, cigarette end, chewing gum and toothbrush and contact items such as collars and headwear give high percentages of full profiles.

When regarding all categories, the five most promising samples to select are muscles, blood, coat collars, cigarette ends and balaclavas. On the other end of the spectrum, the five least promising samples are hairs, plastic bags, bullets, touch traces various and grip traces various. Importantly, for all categories full and usable profiles are obtained. For the sampled item bag plastic for instance 44% of the samples categorize into ‘no  profile’ while 43% may result in a full profile.

The category ‘partial profile possibly useful’ presents uncertainty as at least a partial profile is expected but it is difficult to predict whether DNA results will be usable for comparison studies. Aspects such as the number of contributors to a profile and mixture ratios will have a role here. Notwithstanding, this collaborative study gives insight in the DNA results of the several traces and may assist crime scene investigators in their decision-making in which many other aspects such as the context of an item in to crime are relevant too.

… To continue reading the full article by Anna A. Mapes, please click here.

SOURCE: This article was first published by Forensic Magazine on 20 October 2015.

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