Trace DNA Transfer in Co-Working Spaces: The Importance of Background DNA Analysis.

The presence of background DNA (bgDNA) can hinder the evaluation of DNA evidence at the activity level, especially when the suspect is expected to be retrieved due to their habitual occupation of the investigated environment. Based on real-life casework circumstances, this study investigates the prevalence, composition, origin, and probable transfer routes of bgDNA found on personal items in situations where their owner and person of interest (POI) share the same workspace. Baseline values of bgDNA were evaluated on the participants' personal items. Secondary and higher degree transfer scenarios of non-self DNA deposition were also investigated. The DNA from co-workers and co-inhabiting partners can be recovered from an individual's personal belongings. Non-self DNA present on the hands and deposited on a sterile surface can generate uninformative profiles. The accumulation of foreign DNA on surfaces over time appears to be crucial for the recovery of comparable profiles, resulting in detectable further transfer onto other surfaces. For a thorough evaluation of touch DNA traces at the activity level, it is necessary to collect information not only about DNA transfer probabilities but also about the presence of the POI as part of the 'baseline' bgDNA of the substrates involved.

Determining the number and size of background samples derived from an area adjacent to the target sample that provide the greatest support for a POI in a target sample.

When sampling an item or surface for DNA originating from an action of interest, one is likely to collect DNA unrelated to the action of interest (background DNA). While adding to the complexity of a generated DNA profile, background DNA has been shown to aid in resolving the genotypes of contributors in a targeted sample, and where references of donors to the background DNA are not available, strengthen the LR supporting a person of interest contributing to the targeted sample. This is possible thanks to advances in probabilistic genotyping, where forensic labs are able to deconvolute complex DNA profiles to obtain lists of genotypes and their associated weights. Coupled with DBLR™, one can then compare multiple evidentiary profiles to each other to determine the contribution of common, but unknown, contributors. Here, we consider factors associated with taking background samples and whether one should collect multiple background samples that all relate to a single target sample, or if one should collect larger background samples rather than smaller samples. Background samples consisted of DNA accumulated on the items primarily by one or both occupants of a single household, while targeted samples were generated from touch deposits, or saliva deposits that had been left to air dry. Samples were collected from areas of various sizes, consisting of only the background, the target and the background directly beneath it, and the target and additional surrounding background. A broad range of DNA quantities were recovered, with larger background samples (400 cm2) yielding significantly more DNA than smaller background samples (30 cm2). Significant differences in DNA quantities between target samples were not observed. Generated DNA profiles were interpreted using STRmix™ and DBLR™, and where there was support for a common donor between the background and target sample, pairwise comparisons were performed to observe the effect on the LR supporting the target DNA donor contributing to the targeted sample when conditioning on one (or two) common donor between the targeted sample and 1-8 background samples. Multiple background samples gave significantly higher LRs compared to a single background sample, the larger sampled background area resulted in larger LR gains than the smaller areas, and four or more background samples reduced LR variability considerably. Here we provide recommendations for the minimum and ideal number of additional background samples that should be collected, and that several smaller samples may be more beneficial than a single larger sample.

Investigative use of human environmental DNA in forensic genetics.

Individuals leave behind traces of their DNA wherever they go. DNA can be transferred to surfaces and items upon touch, can be released into the air, and may be deposited in indoor dust. The mere presence of individuals in a location is sufficient to facilitate either direct or indirect DNA transfer into the surrounding environment. In this study, we analyzed samples recovered from commonly touched surfaces such as light switches and door handles in an office environment. We evaluated two different methods to isolate DNA and co-extract DNA and RNA from the samples. DNA profiles were compared to the references of the inhabitants of the different locations and were analyzed taking into consideration the type of sampled surface, sampling location and information about the activities in a room during the sampling day. Results from DNA samples collected from surfaces were also compared to those from air and dust samples collected in parallel from the same areas. We characterized the amount and composition of DNA found on various surfaces and showed that surface DNA sampling can be used to detect occupants of a location. The results also indicate that combining information from environmental samples collected from different DNA sources can improve our understanding of DNA transfer events in an indoor setting. This study further demonstrates the potential of human environmental DNA as an investigative tool in forensic genetics.

Evaluation of background DNA damage in a Turkish population measured by means of the cytokinesis-block micronucleus cytome assay.

This study aimed to determine the background incidence of micronuclei (MN), nucleoplasmic bridges (NPBs), and nuclear buds (NBUDs) in the peripheral blood lymphocytes of a population in the province of Çanakkale, Turkey. Seventy-one individuals living under similar socio-economic conditions, not exposed to an X-ray examination during the last six months, receiving no medical treatment and not involved in agricultural activities were evaluated. In addition to genetic end-points, the cytokinesis-block proliferation index (CBPI) was determined. MN and NBUD frequencies gradually increased with age. MN frequencies of the older group were ∼7 fold higher than those of the younger group. Multiple linear regression analysis showed that apart from age, no significant effects of various confounding factors on MN and NBUD were observed. NPB frequencies were affected by gender. In the present study, no statistically significant effect was found of smoking on the genetic end-points evaluated, which agrees with results from other studies.

Human background DNA on stones in an urban environment.

Stones are frequently used as tools in criminal acts. In our department, around 5 % of all analysed crime scene related trace samples are contact or touch DNA traces swabbed from stones. These samples are primarily related to cases of damage to property and burglary. In court, questions can arise about DNA transfer and the persistence of background DNA not related to the respective crime. To shed some light on the question of how likely it is to detect human DNA as background DNA on stones from an urban environment, the surfaces of 108 stones sampled throughout the city of Bern, the Swiss capital, were swabbed. We detected a median quantity of 33 pg on the sampled stones. STR-profiles suitable for a CODIS (Combined DNA Index System) registration in the Swiss DNA database were established from 6.5 % of all sampled stone surfaces. For comparison, retrospective casework data analysis from routine crime scene samples demonstrates a success rate of 20.6 % for the establishment of CODIS-suitable DNA profiles from stones sampled for touch DNA. We further investigated how climatic conditions, location and properties of the stones affected the quantity and quality of the recovered DNA. In this study, we show that the quantity of the measurable DNA decreases significantly with increasing temperature. Furthermore, less DNA could be recovered from porous stones, compared to smooth ones.

Background levels of body fluids and DNA on the shaft of the penis and associated underpants in the absence of sexual activity.

This study examines the background of blood, saliva, semen and autosomal DNA on penile swabs and underpants from males in the absence of recent sexual activity. Based on the data collected by the AFSP Body Fluid Forum, the results of this study show that; there is a very low expectation of detecting blood on penile swabs and male underpants; a low expectation of detecting saliva on penile swabs and male underpants; and spermatozoa would be expected in less than a quarter of penile swabs and three quarters of male underpants. As none of the samples had detectable levels of DNA which were suitable for meaningful comparison that did not match the donor or their partner, the expectation of detecting a DNA profile from the cellular background on penile swabs or underpants from a male who has not been involved in recent sexual intercourse is very low. The results of this study are extremely informative when evaluating the significance of blood, saliva, semen and DNA detected on the penile swabs and underpants of males in cases of alleged sexual assault.

Exploring how the LR of a POI in a target sample is impacted by awareness of the profile of the background derived from an area adjacent to the target sample.

DNA unrelated to an action of interest (background DNA) is routinely collected when sampling an area for DNA that may have originated from an action of interest. Background DNA can add to the complexity of a recovered DNA profile and could impact the discrimination power when comparing it to the reference profile of a person of interest. Recent advances in probabilistic genotyping and the development of new tools, now allow for the comparison of multiple evidentiary profiles to query for a common DNA donor. Here, we explore the additional discrimination power that can be gained by having an awareness of the background DNA present on a surface prior to the deposition of target DNA. Samples with varying number of contributors and DNA quantities were generated on cleaned plastic pipes (where ground truth was known) and items used by occupants of a single household (where ground truth was not known). The background consisted of deposits made by hands (touch) while target deposits were both touch and saliva. Samples were collected from areas consisting of only the background (A), the target and the background directly beneath it (B), and the target and additional surrounding background (B+C). Samples B and B+C yielded similar DNA amounts when the target consisted of saliva, but when the target consisted of touch, significantly more DNA was recovered from B+C. Subsequently generated DNA profiles were interpreted using STRmix™ and DBLR™. The first approach involved no conditioning while the second approach involved conditioning on the reference profiles of the known background DNA donors. The third approach involved conditioning on one common DNA donor between A and B or A and B+C. The fourth and final approach involved conditioning on two common DNA donors between A and B or A and B+C. As more information was applied to the analysis, the greater the increase in the LR for the comparison of the target sample to the POI. Conditioning on two common donors between the target and the background provided almost the same amount of information as conditioning on the references of the known background DNA donors. This resulted in an increase in the LR that was over 10 orders of magnitude for known donors in the target sample. Here we have demonstrated the value in collecting additional background samples from an area adjacent to a targeted sample, and that this has the potential to improve discrimination power.

Direct and Secondary Transfer of Touch DNA on a Credit Card: Evidence Evaluation Given Activity Level Propositions and Application of Bayesian Networks.

In a judiciary setting, questions regarding the mechanisms of transfer, persistence, and recovery of DNA are increasingly more common. The forensic expert is now asked to evaluate the strength of DNA trace evidence at activity level, thus assessing if a trace, given its qualitative and quantitative features, could be the result of an alleged activity. The present study is the reproduction of a real-life casework scenario of illicit credit card use by a co-worker (POI) of its owner (O). After assessing the shedding propensity of the participants, differences in DNA traces' qualitative and quantitative characteristics, given scenarios of primary and secondary transfer of touch DNA on a credit card, a non-porous plastic support, were investigated. A case-specific Bayesian Network to aid statistical evaluation was created and discrete observations, meaning the presence/absence of POI as a major contributor in both traces from direct and secondary transfer, were used to inform the probabilities of disputed activity events. Likelihood Ratios at activity level (LRα) were calculated for each possible outcome resulting from the DNA analysis. In instances where only POI and POI plus an unknown individual are retrieved, the values obtained show moderate to low support in favour of the prosecution proposition.

DNA transfer between worn clothing and flooring surfaces with known histories of use.

DNA samples recovered from items of clothing are often attributed to the wearer and one or more individuals who may have contacted the item during an alleged criminal activity. Another scenario often proposed by defence counsel is that DNA was transferred from a previously contacted item/surface unrelated to the activity of interest onto the item of clothing. Under such scenarios, DNA may also be transferred from the clothing to the item/surface with which it comes into contact. One such surface is flooring, upon which clothing may be placed while not being worn or may be contacted during wearing, such as falling or being forced to the ground. This study investigates the transfer of DNA to and from clothing and flooring when different contacts are applied between the two surfaces in an environment representative of what investigators would encounter in routine casework, a residential environment. Participants were provided with two sets of new and unused upper and lower garments to wash then wear for ~8 h inside their own home before storing them in paper evidence bags. The two sets of clothing were taken to a home occupied by unrelated individuals, where one set was placed on the floor ('passive') by the researcher while the other was worn by the participant who laid with their back on the floor, rolled to one side and back, then stood up ('active'). Within the houses sampled, the main bedroom was targeted as flooring types and histories of use were more consistent across houses and less variation in DNA profile composition was previously observed for samples collected in the same room. Samples were collected from predetermined areas of the clothing and flooring where contact did and did not occur. Reference profiles were obtained from wearers and individuals they lived with, as well as occupants of the home. DNA transfer was observed from clothing to flooring and from flooring to clothing in both 'active' and 'passive' situations, though greater where a situation involved the application of pressure and friction ('active'), and only where contact between clothing and flooring occurred. Results from this study inform on the composition of DNA profiles one is likely to obtain from an item of clothing or a flooring surface following a similar contact event between the two substrates and will aid investigators when interpreting DNA evidence recovered in a domestic environment and the activities leading to its transfer and subsequent recovery.

The importance of considering common sources of unknown DNA when evaluating findings given activity level propositions.

Evaluating forensic biological evidence considering activity level propositions is becoming more prominent around the world. In such evaluations it is common to combine results from multiple items associated with the alleged activities. The results from these items may not be conditionally independent, depending on the mechanism of cell/DNA transfer being considered and it is important that the evaluation takes these dependencies into account. Part of this consideration is to incorporate our understanding of prevalent DNA and of background DNA on objects and people, and how activities can lead to common sources of unknown DNA being deposited on items. We demonstrate a framework for evaluation of DNA evidence in such a scenario using Object-Oriented Bayesian Networks and apply it to a motivating case from South Australia.

Investigating the position and level of DNA transfer to undergarments during digital sexual assault.

Sexual assault cases are regularly encountered in forensic laboratories. The victim's underwear and other clothes are often submitted with the intention of retrieving the offender's DNA. We examined six sexual assault scenarios; removing underwear, removing brassiere, digital penetration of the vagina from the front, grabbing breasts over the top of the brassiere, digital penetration of the vagina from the rear, and grabbing the breasts under the brassiere from the rear. For each scenario we used a dummy weighing approximately 45 kg to mimic a victim and recorded where the underwear was contacted and how much DNA was transferred using volunteer 'offenders'. We found that how much DNA was recovered depended on which individual acted as offender and suggest it depends on the shedder status of the offender. We also discovered instances of the spouse, or office co-worker of the offender on the underwear. The obtained results will assist in identifying areas to target on underwear for different sexual assault scenarios, and also will provide important information that can be used for assigning probabilities of DNA transfer aiding evaluations of forensic biology findings that consider activity level propositions.

A Study of the Background Levels of Male DNA on Underpants Worn by Females.

This study is the first to examine the background level of male DNA on underpants worn by females in the absence of sexual contact. Here, we examined 103 samples from the inside front of underpants from 85 female volunteers. Samples were examined for the presence of male DNA using NGM SElect and PowerPlex Y23 kits. Only five samples gave a "complete" Y-STR profile, even though 83.5% of our volunteers cohabited with a male. In all cases where a partner reference sample was available, the Y-STR profile matched the cohabiting partner. We have demonstrated that a Y-STR profile is not expected on the inside front of underpants worn by females after social contact alone. The results of this study are informative for evaluating the significance of a Y-STR profile on underpants in cases of alleged sexual assault.

Using Bayesian networks to track DNA movement through complex transfer scenarios.

Trace DNA and the manner in which it is transferred from item to item is a common topic arising in forensic science, both in case evaluations, and in Court testimony. In order to assign the probability of obtaining DNA findings, given competing propositions that specify transfer mechanisms, consideration must be given to a number of factors. Previous work by the authors developed a simple Object-Oriented Bayesian Network (OOBN) that pooled numerous published studies in order to attempt evaluation of trace DNA results given such propositions. In this work, we expand on the previously published OOBN and formalise a number of class networks that can be used together in a logical way to consider DNA movement through complex chains of transfer events. Specifically, we develop an OOBN that considers the two-way transfer of DNA that occurs when two items contact, and allows for the sampling of intermediary items involved in the chain of transfers. The aim is to show that adopting an approach involving basic building blocks, we offer the possibility to tackle complex and various cases for which the OOBN will be obtained by combining their elementary blocks. We conclude with a demonstration of applying the OOBN being applied to a chain of transfers in a case scenario.

Background levels of male DNA in the vaginal cavity.

In alleged sexual assault investigations, the detection of male DNA on female intimate swabs can be established through Y-STR profiling. However, in cases where the defendant lives in the same dwelling as the complainant, the defendant may argue that his DNA was transferred through normal social contact, and dispute that intimate contact occurred. This study aimed to determine whether or not male DNA could be detected in samples collected from the vaginal cavity after everyday activities. A total of 300 samples from eight individuals were subjected to DNA quantification and Y-STR amplification. Three donors reported sexual contact during the time of sampling, hence the detection of male-specific DNA in post-coital samples was concomitantly evaluated. The majority of samples (n=237, 79%) were undetermined for male DNA by real-time PCR quantification. No Y-STR profiles having three or more alleles were obtained in these samples, with the exception of three samples which displayed three alleles each. One sample was collected six days post-coital, one sample likely presented artefacts, and one exhibited two alleles at the same locus. Male-specific quantification values were obtained for the remaining 63 samples (21%), however only 31 samples displayed alleles at three or more loci. Twenty-three of the 63 samples showed inhibition during real-time PCR quantification, and the majority of these exhibited no Y-STRs. A further three samples were excluded from the dataset due to possible contamination. The maximum confirmed time frame for obtaining a full Y-STR profile was six days post-coital, but may be longer in some donors. Overall concordance between DNA quantification and Y-STR amplification was 85.8%. This study demonstrates that obtaining a Y-STR profile comprising three or more alleles from a vaginal swab collected from a woman who has not participated in recent intimate sexual activities is not indicated. This suggests that there is unlikely to be detectable adventitious transfer of DNA from male individuals living in the same household as a female donor. However, approximately one in seven samples may display 1-2 unexpected allelic peaks (<400 RFU). These are unlikely to originate from the male partner or cohabitant of the donor and should be interpreted with extreme caution, and with every effort attempted to replicate results before a conclusion is derived about their relevance. These findings support the significance of male DNA profiles consisting of more than three alleles obtained from female intimate samples during investigations of alleged sexual assault.

Detection of offender DNA following skin-to-skin contact with a victim.

Forensic medical examiners are frequently asked to examine persons who claim to have been assaulted. If the suspect is unknown and there has been contact between his or her skin and the alleged victim, there is an expectation that DNA can be collected from the victim's skin. In this way, the retrieved suspect DNA from the skin of the victim can be used to support the proposition that places the suspect at the scene. This study investigated the transfer and persistence of offender DNA on a victim following a mock physical assault and further transfer of the offender DNA onto clothing worn over the assaulted area. Mock assault scenarios were conducted with the offender using medium pressure without friction and heavy pressure with friction on the wrist and upper arm of the victim. Samples were taken at either 0 h, 3 h or 24 h post assault, with 18 assault scenarios conducted at each time point. Samples from the victim's skin where the assault had taken place and, where applicable, clothing worn over the assaulted area were collected using the double swabbing method. Offender DNA was observed on the victim's skin at 0 h in the majority of samples with a higher transfer rate observed where heavy pressure and friction was used. The presence of the offender profile was detected in the samples collected at 3 h and 24 h, with 25% and 12% of samples respectively producing a LR in support of Hp. Transfer of offender DNA to clothing worn over the assaulted area was demonstrated, with 19% of samples producing a LR in support of Hp. Samples taken from clothing were complex mixtures, with 23% of samples producing four or more person mixtures. Indirect transfer of other DNA was also observed, with background DNA on the offender's skin observed in the clothing swabs of the victim on a number of occasions. Our data suggests that sampling from clothing worn over the assaulted area may be an additional or better avenue for the recovery of offender DNA post assault where there has been significant time between assault and sampling.

The implications of shedder status and background DNA on direct and secondary transfer in an attack scenario.

In court questions are often raised related to how trace DNA was deposited, directly during the crime or innocently for instance by secondary transfer. It is therefore of interest to have knowledge of the probability of transfer or secondary transfer in different situations. Factors that could influence transfer probabilities are background DNA and the shedder status of the involved persons. In this study, we have classified participants as high or low DNA shedders. We observed DNA transfer in a simulated attack scenario, and demonstrated that shedder status has a significant influence of transfer rates. We have examined the background DNA in samples from T-shirts worn in an area with frequent human traffic and detected multiple contributors. We further demonstrated that DNA from co-workers of a T-shirt wearer can be secondarily transferred from the environment and detected in samples, and that the composition of background DNA is correlated with the shedder status of the wearer. Finally, we have illustrated the inference with the results of transfer probabilities and a fictive case with the use of a Bayesian network.

Probability of detection of DNA deposited by habitual wearer and/or the second individual who touched the garment.

Available literature on the detection of transferred DNA does not address the interpretation issues in relation to who wore rather than touched the garment. To acquire a greater knowledge of the rate of detectable wearer, toucher and background DNA, 63 males wore their own underpants for 12h. The inside-waistband was handled by one of 11 female volunteers for 15s. The waist-band was mini-taped and subjected to DNA profiling with the AMPFℓSTR(®) NGM SElect™ kit. The findings show that on worn garments the probability of observing reportable DNA profiles is 61.9%. The wearer was detected as a single profile or part of a mixed profile in 50.8% of samples. When the wearer was present in a mixture, he was always observed as the major contributor. The toucher was detected on 11.1% of underpants. Reportable background DNA (non-wearer and non-toucher) occurs in 14.3% of samples and may affect the assessment of who wore the garment. Greater knowledge of the frequency of detection of reportable wearer DNA and/or toucher allows scientists to evaluate the likelihood of observing a matching profile if an individual wore a garment rather than touched it in disputed case scenarios.

Following the transfer of DNA: How does the presence of background DNA affect the transfer and detection of a target source of DNA?

DNA transfer is of increasing importance in crime scene situations, partly due to analytical techniques detecting profiles in ever declining amounts of DNA. Whereas the focus has previously been DNA transfer of target sources, the effects of background DNA on transfer and detection of DNA after multiple contact situations have been much less investigated. This study measured the transfer and detection rates of a specific DNA source in the presence of background DNA sources. The presence of background DNA influenced the transfer of DNA differently depending on the combination of biological material and surface type. The detection of a profile from the target DNA decreased after multiple contact situations, due to the reduced total and relative quantity of target DNA, and the increasing complexity of the mixture. The results of this study contribute to a greater understanding of the effects of background DNA sources on DNA transfer and detection.