Bianca Szkuta

DNA transfer by examination tools--a risk for forensic casework?

The introduction of profiling systems with increased sensitivity has led to a concurrent increase in the risk of detecting contaminating DNA in forensic casework. To evaluate the contamination risk of tools used during exhibit examination we have assessed the occurrence and level of DNA transferred between mock casework exhibits, comprised of cotton or glass substrates, and high-risk vectors (scissors, forceps, and gloves). The subsequent impact of such transfer in the profiling of a target sample was also investigated. Dried blood or touch DNA, deposited on the primary substrate, was transferred via the vector to the secondary substrate, which was either DNA-free or contained a target sample (dried blood or touch DNA). Pairwise combinations of both heavy and light contact were applied by each vector in order to simulate various levels of contamination. The transfer of dried blood to DNA-free cotton was observed for all vectors and transfer scenarios, with transfer substantially lower when glass was the substrate. Overall touch DNA transferred less efficiently, with significantly lower transfer rates than blood when transferred to DNA-free cotton; the greatest transfer of touch DNA occurred between cotton and glass substrates. In the presence of a target sample, the detectability of transferred DNA decreased due to the presence of background DNA. Transfer had no impact on the detectability of the target profile, however, in casework scenarios where the suspect profiles are not known, profile interpretation becomes complicated by the addition of contaminating alleles and the probative value of the evidence may be affected. The results of this study reiterate the need for examiners to adhere to stringent laboratory cleaning protocols, particularly in the interest of contamination minimisation, and to reduce the handling of items to prevent intra-item transfer.

Shedder status—An analysis of self and non-self DNA in multiple handprints deposited by the same individuals over time

There are several studies that suggest that different people deposit different quantities of their own DNA on items they touch, i.e. some are good shedders and others are bad shedders. It is of interest to determine if individuals deposit consistent quantities of their own DNA, no matter the occasion, as well as the degree of variability among individuals. To investigate this, participants were tested for their ability to deposit DNA by placing right and left handprints on separate DNA-free glass plates at three set times during the day (morning, midday and afternoon) on four different days spaced over several weeks. Information regarding recent activities performed by the individual was recorded, along with information on gender, hand dominance and hand size. A total of 240 handprint deposits were collected from 10 individuals and analyzed for differences in DNA quantity and the type of the DNA profile obtained at different times of the day, on different days, between the two hands of the same individual, and between different individuals. Furthermore, the correlation between the deposit quantity and the ratio of self to non-self DNA in the mixed deposits was analyzed to determine if the amount of non-self DNA has an effect on overall DNA quantities obtained. In general, this study has shown that while there is substantial variation in the quantities deposited by individuals on different occasions, some clear trends were evident with some individuals consistently depositing significantly more or less DNA than others. Non-self DNA was usually deposited along with self DNA and, in most instances, was the minor component. Incidents where the non-self portion was the major component were very rare and, when observed, were associated with a poor depositor/shedder. Forensic DNA scientists need to consider the range and variability of DNA a person deposits when touching an object, the likelihood of non-self DNA being co-deposited onto the handled object of interest and the factors that may affect the relative quantity of this component within the deposit.

Transfer and persistence of DNA on the hands and the influence of activities performed

During the evaluation of forensic DNA evidence in court proceedings, the emphasis previously placed on the source of the DNA is progressively shifting to the consideration of the activities resulting in its deposition. While direct contact and deposition may be a likely explanation, alternative scenarios involving DNA transfer through a secondary person or medium are important to consider. Here we assessed whether non-self DNA, indirectly transferred via a handshake, could be detected on surfaces contacted by the opposing hand-shaker after 15min, and considered the variables affecting its persistence in subsequent contacts. In general, the depositor of the handprint was the major contributor to DNA profiles collected from handprints placed on glass plates. Minor contributions from the opposing hand-shaker (as a known contributor) were detected at a lower rate, decreasing as the number of contacted items increased post-handshake. Delays in deposition also affected the detection of the opposing hand-shaker, with a 15min delay between handshaking and contact resulting in the reduced presence, and corresponding LRs, of the known contributor. The handprint depositor was excluded from their own handprint on several occasions, including instances where the opposing hand-shaker was not excluded from the same profile. Several factors appeared to strongly influence the detection of both the depositor and contributing individual involved in the handshake. The relative shedding ability of the pair had the largest effect, where good shedders (whether depositor or contributor) could swamp poor to moderate shedders, while the pairing of two moderate or two poor shedders could result in the detection of both individuals. When the deposition of a handprint was delayed, the activities performed by the individual had a substantial effect on the resultant detection of the contributing profile - multiple contacts with the same items increased the likelihood that the known contributors DNA would be retained and subsequently detected, through the parking and re-transfer of DNA on used items.

An inter-laboratory comparison study on transfer, persistence and recovery of DNA from cable ties

To address questions on the activity that led to the deposition of biological traces in a particular case, general information on the probabilities of transfer, persistence and recovery of cellular material in relevant scenarios is necessary. These figures may be derived from experimental data described in forensic literature when conditions relevant to the case were included. The experimental methodology regarding sampling, DNA extraction, DNA typing and profile interpretation that were used to generate these published data may differ from those applied in the case and thus the applicability of the literature data may be questioned. To assess the level of variability that different laboratories obtain when similar exhibits are analysed, we performed an inter-laboratory study between four partner laboratories. Five sets of 20 cable ties bound by different volunteers were distributed to the participating laboratories and sampled and processed according to the in-house protocols. Differences were found for the amount of retrieved DNA, as well as for the reportability and composition of the DNA profiles. These differences also resulted in different probabilities of transfer, persistence and recovery for each laboratory. Nevertheless, when applied to a case example, these differences resulted in similar assignments of weight of evidence given activity-level propositions.

Transfer and persistence of non-self DNA on hands over time: Using empirical data to evaluate DNA evidence given activity level propositions

Questions relating to how DNA from an individual got to where it was recovered from and the activities associated with its pickup, retention and deposition are increasingly relevant to criminal investigations and judicial considerations. To address activity level propositions, investigators are typically required to assess the likelihood that DNA was transferred indirectly and not deposited through direct contact with an item or surface. By constructing a series of Bayesian networks, we demonstrate their use in assessing activity level propositions derived from a recent legal case involving the alleged secondary transfer of DNA to a surface following a handshaking event. In the absence of data required to perform the assessment, a set of handshaking simulations were performed to obtain probabilities on the persistence of non-self DNA on the hands following a 40min, 5h or 8h delay between the handshake and contact with the final surface (an axe handle). Variables such as time elapsed, and the activities performed and objects contacted between the handshake and contact with the axe handle, were also considered when assessing the DNA results. DNA from a known contributor was transferred to the right hand of an opposing hand-shaker (as a depositor), and could be subsequently transferred to, and detected on, a surface contacted by the depositor 40min to 5h post-handshake. No non-self DNA from the known contributor was detected in deposits made 8h post-handshake. DNA from the depositor was generally detected as the major or only contributor in the profiles generated. Contributions from the known contributor were minor, decreasing in presence and in the strength of support for inclusion as the time between the handshake and transfer event increased. The construction of a series of Bayesian networks based on the case circumstances provided empirical estimations of the likelihood of direct or indirect deposition. The analyses and conclusions presented demonstrate both the complexity of activity level assessments concerning DNA evidence, and the power of Bayesian networks to visualise and explore the issues of interest for a given case.

DNA transfer in forensic science: a review

Understanding the variables impacting DNA transfer, persistence, prevalence and recovery (DNA-TPPR) has become increasingly relevant in investigations of criminal activities to provide opinion on how the DNA of a person of interest became present within the sample collected. This review considers our current knowledge regarding DNA-TPPR to assist casework investigations of criminal activities. There is a growing amount of information available on DNA-TPPR to inform the relative probabilities of the evidence given alternative scenarios relating to the presence or absence of DNA from a specific person in a collected sample of interest. This information should be used where relevant. However, far more research is still required to better understand the variables impacting DNA-TPPR and to generate more accurate probability estimates of generating particular types of profiles in more casework relevant situations. This review explores means of achieving this. It also notes the need for all those interacting with an item of interest to have an awareness of DNA transfer possibilities post criminal activity, to limit the risk of contamination or loss of DNA. Appropriately trained forensic practitioners are best placed to provide opinion and guidance on the interpretation of profiles at the activity level. However, those requested to provide expert opinion on DNA-related activity level issues are often insufficiently trained to do so. We advocate recognition of DNA activity associated expertise to be distinct from expertise associated with the identification of individuals. This is to be supported by dedicated training, competency testing, authorisation, and regular fit for purpose proficiency testing. The possibilities for experts to report on activity-related issues will increase as our knowledge increases through further research, access to relevant data is enhanced, and tools to assist interpretations are better exploited. Improvement opportunities will be achieved sooner, if more laboratories and agencies accept the need to invest in these aspects as well as the training of practitioners.

Sharing data on DNA transfer, persistence, prevalence and recovery: Arguments for harmonization and standardization

Sharing data between forensic scientists on DNA transfer, persistence, prevalence and recovery (TPPR) is crucial to advance the understanding of these issues in the criminal justice community. We present the results of a collaborative exercise on reporting forensic genetics findings given activity level propositions. This exercise outlined differences in the methodology that was applied by the participating laboratories, as well as limitations to the use of published data on DNA TPPR. We demonstrate how publication of experimental results in scientific journals can be further improved to allow for an adequate use of these data. Steps that can be taken to share and use these data for research and casework purposes are outlined, and the prospects for future sharing of data through publicly accessible databases are discussed. This paper also explores potential avenues to proceed with implementation and is intended to fuel the discussion on sharing data pertaining to DNA TPPR issues. It is further suggested that international standardization and harmonization on these topics will benefit the forensic DNA community as it has been achieved in the past with the harmonization of STR typing systems.

DNA decontamination of fingerprint brushes

Genetic profiling of DNA collected from fingerprints that have been exposed to various enhancement techniques is routine in many forensic laboratories. As a result of direct contact with fingermark residues during treatment, there is concern around the DNA contamination risk of dusting fingermarks with fingerprint brushes. Previous studies have demonstrated the potential for cross-contamination between evidentiary items through various mechanisms, highlighting the risk of using the same fingerprint brush to powder multiple surfaces within and between crime-scenes. Experiments were performed to assess the contamination risk of reused fingerprint brushes through the transfer of dried saliva and skin deposits from and to glass surfaces with new unused squirrel hair and fiberglass brushes. Additional new unused brushes and brushes previously used in casework were also tested for their ability to contaminate samples. In addition, the ability to eradicate DNA from used squirrel hair and fiberglass fingerprint brushes was assessed using a 1% sodium hypochlorite solution and a 5% solution of a commercially available alternative, Virkon. DNA profiling results from surfaces contacted by treated and untreated brushes were compared to determine the effectiveness of the devised cleaning protocol. Brush durability was also assessed over multiple wash/rinse/dry cycles with both agents. Varying amounts of DNA-containing material were collected and transferred by squirrel hair and fiberglass brushes, with detectability on the secondary surface dependent on the biological nature of the material being transferred. The impact of DNA contamination from dirty fingerprint brushes was most apparent in simulations involving the transfer of dried saliva and brushes previously used in casework, while minimal transfer of touch DNA was observed. Alarmingly, large quantities of DNA were found to reside on new unused squirrel hair brushes, while no DNA was detected on new unused fiberglass brushes or brushes sold as DNA-free. Squirrel hair brushes were easily and effectively cleaned with both hypochlorite and Virkon, with no evidence of DNA transfer between exhibits by treated brushes. Brushes were still deemed useable after multiple cleaning cycles with either agent. In contrast, fiberglass bristles became tangled and matted when wet and could not be cleaned effectively using either method. It is recommended they are disposed of following use. Each laboratory should consider their current circumstances before adapting a cleaning method. The implementation of a program to monitor the effectiveness of the cleaning regime is also advised.

Trace DNA Profiling in Missing Persons Investigations

The sources of sufficient quantities of DNA to provide profiles useful in assisting missing persons investigations have expanded over recent years due to improvements in DNA recovery and profiling methodologies. The collection and processing of trace samples should, however, only be contemplated when traditional high quantity and quality samples are not available. Whilst potentially useful in many situations, a trace sample may require application of alternative profiling methodologies, be less likely to provide a full profile, be more likely to be part of a mixed DNA sample that is difficult to deconvolute thus reducing match probabilities, and require greater care during collection and processing to limit DNA contamination risks and impacts.