Evaluation of multiple transfer of DNA using mock case scenarios

Article Outline

• Abstract
• 1. Introduction
• 2. Materials and methods
  • 2.1. Case 1
    • 2.1.1. Proposed events
    • 2.1.2. Experiment design
  • 2.2. Case 2
    • 2.2.1. Proposed events
    • 2.2.2. Experimental design
  • 2.3. Case 3
    • 2.3.1. Proposed events
    • 2.3.2. Experimental design
  • 2.4. Sample processing
  • 2.5. Data analysis
  • 2.6. Quality control
• 3. Results
  • 3.1. Case 1
    • 3.1.1. DNA quantities
    • 3.1.2. DNA profiles
    • 3.1.3. Transfer percentages
  • 3.2. Case 2
    • 3.2.1. DNA profiles
  • 3.3. Case 3
    • 3.3.1. DNA quantities
    • 3.3.2. DNA profiles
    • 3.3.3. Transfer percentages
• 4. Discussion
  • 4.1. Case 1
  • 4.2. Case 2
  • 4.3. Case 3
  • 4.4. General
• References
• Copyright

Abstract 

DNA transfer and its possible role in explaining the presence of a biological sample at a crime scene is becoming more prevalent in criminal investigations and related court proceedings. To assist understanding of DNA transfer and assess the extent to which we can utilise already available information regarding transfer of DNA we compare transfer rates determined from mock multi-step transfer scenarios with transfer rates predicted by the application of currently available transfer rate data. The transfer results obtained from the scenarios tested were, in some instances, different (both lower and higher rates) from those predicted. These discrepancies are most likely the result of the impact of as yet untested variables. These may include the variations in substrate type, transfer area size and environmental factors such as temperature and humidity among others. Whilst detailed re-enactments of proposed transfer scenarios, that take into account the many possibly relevant aspects affecting transfer are desirable, to provide an accurate likelihood estimate, these are not always possible. The application of detailed transfer rate tables that include data on the many factors affecting transfer could provide a useful substitute for evaluating the likelihood of specific transfer events. The value and accuracy derived from applying such tables will improve as more research in this area is conducted and the tables expanded and refined.

Keywords: Forensic science, Transfer, DNA, Trace, Crime scene, DNA typing

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1. Introduction 

While the concept of secondary transfer occurring during illegal activity has been part of criminal investigation since the early 20th century [1], [2], secondary DNA transfer has only recently become an issue in criminal proceedings [3], [4], [5]. The likelihood of such transfer and the factors affecting it remain largely unknown, leaving forensic practitioners vulnerable to the scrutiny of lawyers on this issue. Recent publications [6], [7] have presented the first, albeit simplistic, methodology for estimation of the percentage of blood, saliva and skin DNA transferred under a specific combination of variables, including type of substrate (cotton, plastic and wool), type of contact (passive, pressure and friction) and sample moisture content (wet and dry).

The value of the data obtained by the above methodology is now tested by conducting three experiments, the design of which was suggested by court cases, in which secondary and further transfer events were proposed as an explanation of the accused’s DNA being present at the crime scene. It should be emphasised that the aim in these experiments was not to replicate aspects of the case scenarios as presented in court, but instead take certain components and variables relevant to the issue of transfer in the particular case and investigate them under laboratory conditions. The results obtained from these experiments are compared with those from the blood and skin transfer data published previously [6], [7]. In this way, the value of these published data in estimating the likelihood of the proposed transfer events can be evaluated. Further, similarities between observed and expected transfer rates would strengthen the application of the latter. Additionally, such comparison between the expected and observed results may reveal the existence of other, unknown, variables that can have a significant influence on transfer that require further investigation. The research findings presented here are by no means exhaustive of the issue, but may offer some insight into the complex nature and behaviour of trace DNA under specific and controlled conditions.

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2. Materials and methods 

2.1. Case 1 

A woman was found strangled and partially burned in her house approximately a day after the offence took place. A tape-lift of the lapel of the victim’s pyjamas provided a DNA profile of her ex-partner who claimed he had not been in her house or in contact with the victim for several months. The ex-partner was charged with murder based on DNA as well as other circumstantial evidence.

At trial, defence proposed secondary DNA transfer scenarios to explain the presence of the accused’s DNA on the victim. Transfer possibilities suggested include: (a) from the accused (possibly skin or saliva) to the child’s clothing and then to the victim’s pyjamas and (b) from the accused (possibly skin or saliva) to the child’s toys and then to the victim’s pyjamas.

A single donor (perpetrator) deposited skin cells on five new, sterilized, toy plastic building blocks (representing children’s toys) by rubbing both hands (friction) on all sides of each object for approximately 1min (immediately prior to the experiment). Five individuals (each representing a victim) were each asked to rub all sides of one building block over the designated area (approx. 20cm×30cm) on the front of their laboratory coat (representing victim’s pyjamas) for approximately 1min. Each individual was wearing disposable clean gloves during the experiment. The boundaries of the designated area on the coat were marked with sterile tape to indicate the part used in the experiment. Each individual was wearing his/her own laboratory coat that had been worn for at least 2days prior to the experiment.

The experiment was repeated as described above, but with infant singlets (representing children’s clothing) instead of toys. The singlets were placed on infant-sized, sterilised dolls for easier handling.

Both of the above experiments with toys and singlets were replicated, but with the inclusion of a 24h interval between the skin cell deposit on toys/singlets and the transfer step to the laboratory coats. Toys and singlets with skin deposits were left in a closed hood during the 24h interval.

2.2. Case 2 

At a hot desert location two individuals (A and B) spent approximately an hour together at a work station, where they both potentially used several objects there such as megaphone, telephone, etc. At the end of the hour, the individuals left for their respective homes and approximately an hour later one of the individuals (B) was found dead, allegedly having killed himself with his own gun. In the space of the hour prior to the alleged suicide, individual B performed many activities such as opening and closing doors, changing clothes, etc. Forensic scientists found both individual A’s and B’s DNA on the gun involved in the incident, with individual A being the major contributor. Individual A asserted that at no time did he touch the gun and that the DNA present on the gun was the result of secondary DNA transfer from one of the items touched by both individuals during their one hour together to the victim’s hands (individual B) and then further transfer from there to the gun.

Individual A (perpetrator) deposited skin cells on five sterile plastic water hose triggers (representing megaphone) by handling it (friction) for approximately 1min. Five individuals, representing individual B (victim), none being the depositor, were then asked to handle (friction) one of the trigger hoses each for 1min. After handling the item, individuals were allowed to return to their usual activities for one hour, but asked not to wear gloves or wash hands, after which time they were each given a sterilized plastic gun which they handled for a duration of 1min (friction).

2.3. Case 3 

A husband and wife both had nose bleeds and used the same towel to dry their faces prior to walking the dog in the park. While walking, the wife became tired and stayed behind, while the husband continued to walk with the dog. The wife was then strangled and stabbed. Gloves and the knife found at the crime scene revealed both the husband’s and an unknown person’s DNA. The defence postulated that the husband’s DNA was present on the exhibits due to tertiary or further transfer: the husband’s blood (nosebleed) was transferred to the towel, then to the wife’s face and subsequently to the gloves and the knife used by the perpetrator.

One hundred microlitres of blood from a single donor was applied to a sterile plastic mask (mask 1 – representing husband’s face) [the mask was mounted onto a sterilized box for easy handling] and spread over the area below the nose (mimicking the nosebleed). A face towel was then applied and wiped over the mask for approximately 1min (friction). The bloodied towel was immediately applied and wiped over the second sterile plastic mask (mask 2 – representing wife’s face) for 1min (friction) under the same conditions. The second mask was left in a closed hood (to limit the risk of contamination) for 1h and a volunteer (strangler) was then asked to “strangle” the second mask for 1min (friction), wearing new latex gloves. The volunteer then held a sterile kitchen knife (plastic handle) for an additional 1min (friction). The experiment was conducted with five replicates.

2.4. Sample processing 

The following items were sampled in order to collect any DNA present: Case 1 – The laboratory coats (designated area) and infant singlets (outside front and back) were tape-lifted and the toys double-swabbed (wet and dry) [8]; Case 2 – The handled areas of plastic guns and hose triggers were double-swabbed; Case 3 – Face masks, gloves and knives (handles) were double-swabbed and face towels tape-lifted.

DNA was extracted with 5% Chelex [9]; further concentrated with an Amicon Ultra® YM-30 centrifugal filter (Millipore) (as per manufacturer’s instructions) to approximately 30μl and quantified with Quantifiler^TM Human DNA Quantification Kit and the ABI PRISM^TM 7500 SDS Instrument (as per manufacturer’s instructions). Several samples that contained minimal DNA quantities after quantification were further concentrated to approximately 10μl by air-drying for approximately 24h in a hood prior to amplification. The samples were typed using AmpF/STR Profiler Plus^TM (Applied Biosystems), ABI PRISM 3100® Genetic Analyser (Applied Biosystems) and Gene Mapper^TM ID Software (Applied Biosystems) using standard procedures.

2.5. Data analysis 

An allele was called when the peak height was ⩾50 RFU. Identical alleles at each particular locus for the two separate donors in a case scenario were termed “in common” between the two. Alleles that were different between the two DNA donors for each locus were termed “unique” to each donor. Alleles common to both DNA donors for each particular locus were counted towards both profiles only if the peak height of the shared allele was at least 1.5 times greater (RFU) than the peak height(s) for the unique allele(s) at the same locus.

The total DNA quantity found on each tested surface was separated into donor, victim and unknown fractions using the RFU data. The unique percent RFU contribution from each donor to a particular profile was calculated by adding together all unique peak height RFUs for a particular donor and dividing the resultant value by the total unique RFU value from all donors. These percentage contributions were applied to the common alleles to divide peak height RFU among its donors. The individual donor contributions to each profile were then noted and the percent contributions identified from this and the total RFU for all donors. The DNA contributed by each donor was estimated by applying these percentages to the total DNA amounts retrieved. The total DNA quantity (ng) on each surface was calculated by multiplying the exact volume of a given extract by its concentration as determined by Quantifiler (ng/μl).

The depositor DNA quantities found on each surface were combined to give an overall DNA quantity deposited per test. The percentage transfer from a particular source to each surface was calculated by dividing the amount of DNA derived from a particular source by the sum of the extracts from all the tested surfaces.

The expected transfer was estimated by using secondary DNA transfer rate data described previously [6], [7]. The expected and observed data were compared using the t-test [10], [11].

For Case 2, measurement of DNA quantities and individual contributions were not performed as the total DNA could not be determined due to undetermined DNA loss during the one hour between the handling of the hose trigger and handling of the toy gun. During this one hour, participants would have touched many objects resulting in both loss of DNA acquired during hose trigger handling and introduction of non-experimental DNA from touched objects such as phone, cup, etc.

2.6. Quality control 

Prior to use all plastic toys and dolls (Case 1), new guns and hose triggers (Case 2), masks, knives and boxes used for mask handling (Case 3) were washed in 70% ethanol and 5% hypochlorite. All items were then sterilized under a UV source for 24h. New face towels (Case 3) were sterilized for 24h under UV. All items were periodically rotated during this 24h period to sterilize all sides.

Additional samples collected from the new doll, toy and singlet (Case 1), gun and hose trigger (Case 2), face towels (×2), masks (×2), knifes (×3), boxes (×5) and gloves (×10) (Case 3) not used in the experiments, but washed and sterilized in a identical fashion, were used as controls and processed in an identical manner to the experimental samples. All control samples produced negative DNA results.

Positive and negative controls were implemented during each step of the DNA analysis and all gave results as expected.

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3. Results 

3.1. Case 1 

All but one tested surface contained detectable DNA quantities, ranging from 0.009 to 9.2ng, and were derived from several sources, including perpetrator (donor), victim (recipient) as well as unknown (Table 1). In all but 1 of 20 coats examined, DNA from the original source (donor) was observed (after contact with the vector (singlet or toy)). Total average DNA quantities deposited on the singlets (2.27 and 5.1ng for immediate and delayed (24h) transfer, respectively) were greater than those deposited on toys (1.2 and 1.03ng for immediate and delayed transfer, respectively) but did not reach a level of significance (p<0.05).

Table 1. Case 1: DNA quantities (ng) retrieved from each surface separated into fractions contributed by donor (perpetrator), recipient (victim) as well as unknown.

Experiment	Singlet immediate		Singlet 24h		Toy immediate		Toy 24h
Surface tested	Singlet	Coat	Singlet	Coat	Toy	Coat	Toya	Coat
1
Total	1.5	0.1	7.5	1.2	0	1.1	0.064	1.47
Donor	1.48	0.1	7.34	0.86	0	0.7	–	1.47
Recipient	0	0	0	0.14	0	0.2	–	0
Unknown	0.02	0	0.165	0.2	0	0.2	–	0
2
Total	1.6	0.3	3.3	0.7	0.1	0.37	0.009	0.2
Donor	1.57	0.3	3.3	0	0.09	0.19	–	0.2
Recipient	0	0	0	0.7	0.01	0.19	–	0
Unknown	0.03	0	0	0	0	0	–	0
3
Total	2.8	2.3	1.8	0.9	0.2	0.9	0.45	0.7
Donor	0.9	2.1	0.3	0.45	0.16	0.6	–	0.5
Recipient	1.8	0.05	1.4	0.2	0.04	0.36	–	0
Unknown	0.1	0.15	0.1	0.25	0	0	–	0.2
4
Total	1.7	1.2	9.2	1.2	2.6	1.97	0.38	1.2
Donor	1.67	0.86	8.9	0.8	2.56	1.005	–	0.96
Recipient	0	0.19	0.2	0.4	0	0.63	–	0.24
Unknown	0.03	0.15	0.1	0	0.04	0.34	–	0
5
Total	1.3	1.4	2.9	1.3	0.06	0.97	0.5	0.6
Donor	1.28	1.09	2.88	0.4	0.06	0.62	–	0.6
Recipient	0	0.2	0	0.8	0	0.35	–	0
Unknown	0.02	0.09	0.002	0.1	0	0	–	0
Total Av.	1.8	1.06	4.9	1.1	0.59	1.07	0.19	0.84
Donor Av.	1.38	0.89	4.5	0.5	0.57	0.61	–	0.75
Recipient Av.	0.36	0.088	0.33	0.5	0.01	0.35	–	0.048
Unknown Av.	0.04	0.08	0.07	0.1	0.008	0.11	–	0.04

A number of alleles were found on most surfaces tested (Table 2). On average, for immediate and delayed transfer combined, doll singlets and laboratory coats contained 25 and 27 alleles, respectively. Of the donor alleles found on doll singlets and coats, 92% and 83% were unique, respectively. In comparison, toys and coats on average contained 1.9 and 16 alleles, respectively (p⩽0.001 (singlets versus toys); p=0.554 (singlet coats versus toy coats)). Of the donor alleles found on toys and coats, 87% and 96% were unique, respectively.

Table 2. Case 1: Number of alleles found on each surface including donor (perpetrator), recipient (victim) and unknown alleles.

Experiment	Replicate number	Donor alleles	Donor unique	Recipient alleles	Recipient unique	Unknown alleles	Total
Singlet immediate	1	20	20	0	0	1	21
	2	20	20	0	0	2	22
	3	18	12	20	14	2	40
	4	19	19	0	0	1	20
	5	19	19	0	0	2	21
Coat immediate	1	1	1	0	0	0	1
	2	3	3	0	0	0	3
	3	20	15	10	5	6	36
	4	15	14	8	7	5	28
	5	7	6	2	1	1	10
Singlet 24h	1	20	20	0	0	3	23
	2	13	13	0	0	0	13
	3	14	10	19	15	4	37
	4	20	16	12	8	1	33
	5	20	20	0	0	1	21
Coat 24h	1	20	15	11	6	10	41
	2	0	0	2	2	0	0
	3	16	13	12	9	3	31
	4	17	12	14	9	0	31
	5	17	12	20	15	7	42
Toy immediate	1	0	0	0	0	0	0
	2	5	5	2	2	0	7
	3	4	4	1	1	0	5
	4	13	13	0	0	1	14
	5	2	2	0	0	0	0
Coat immediate	1	16	13	11	8	8	35
	2	5	4	5	4	0	10
	3	10	6	12	8	0	22
	4	15	11	19	15	8	42
	5	8	7	3	12	0	21
Toy 24h	1	0	0	0	0	0	0
	2	0	0	0	0	0	0
	3	0	0	0	0	0	0
	4	0	0	0	0	0	0
	5	0	0	0	0	0	0
Coat 24h	1	15	15	0	0	0	15
	2	1	1	0	0	0	1
	3	5	5	0	0	7	12
	4	12	11	2	1	0	14
	5	10	10	0	0	0	10

DNA from an unknown source was found in 19 of 35 tests including 10 of 20 vectors (toys+singlets) tested and 9 of 20 coats tested. Evaluation of the unknown DNA on both the vector and the coat showed that 55% of alleles were common to both. Comparison of unknown alleles present on all the vectors (same donor) found that 64% were unique while comparison of unknown alleles present on all the coats (different wearers) found 79% were unique.

Immediate and delayed DNA transfer from the singlet (i.e. child’s clothing) to the coat (average of 34% and 20%, respectively) was similar to the expected transfer values (average of 33% and 13%, respectively) (Table 3). Delayed transfer from the toy to the coat differed from the expected (74% and 49%, respectively) while the immediate transfer from the toy to the coat was 5 times greater than that expected (73% and 14%, respectively) (Table 3).

Table 3. Case 1: Expected and observed transfer percentages from each experimental surface to the coats.

Experiment	Expected transfera	Observed transfer
Replicate		1	2	3	4	5	Av.
Singlet (immediate)	33b	6	16	70	34	46	34
Singlet (24h)	13c	11	0	70	8	12	20
Toy (immediate)	14d	100	68	79	28	91	73
Toy (24h)	49e	96	96	53	72	55	74

3.2. Case 2 

Several of the experimental surfaces contained a number of alleles (Table 4).

Table 4. Case 2: Number of alleles found on each surface including those from perpetrator, victim and unknown alleles.

In general, more alleles were found on hose triggers than guns, with an average of 8 and 1.8 alleles, respectively, (p=0.107). On average, 3.2 and 1.2 (individual A) alleles were found on hose triggers and guns, respectively. Of the perpetrator alleles found on hose triggers and guns 62.5% and 83% were unique, respectively. Some victim’s (individual B) alleles were also found on both items (average of 4.4 and 0.6 alleles, respectively, on hose triggers and guns). Of the victim’s alleles found on hose triggers and guns 73% and 67% were unique, respectively. DNA from an unknown source was found in 1 of 10 substrates (2 alleles).

3.3. Case 3 

DNA quantities retrieved from each tested surface are shown in Table 5. On average 0.15 and 0.1ng of DNA was found on the gloves and the knife, respectively.

Table 5. Case 3: DNA quantities (ng) retrieved from gloves and knives separated into fractions^a contributed by donor, vector (strangler volunteer) as well as unknown.

Several alleles were found on the experimental surfaces (Table 6). DNA from an unknown source was found on 5 of 15 surfaces, including 4 of 10 gloves and 1 of 5 knives tested. Comparison of unknown alleles on all items (same donor) revealed that all were unique.

Table 6. Case 3: Number of alleles found on gloves and knives including donor, vector (strangler volunteer) and unknown alleles.

One knife produced alleles after typing, however, no quantifiable DNA was found on that item and, therefore, fractioning was not performed and transfer percentages not calculated. Additionally, although two of the remaining knives contained quantifiable DNA, they did not contain any alleles after typing (except for one which contained one unknown allele) thus preventing further analysis.

The transfer percentages from mask 1 (husband) to the towel and from mask 2 to the gloves were, on average, lower than expected (64% versus 97%; and 9% versus 44.5%, respectively), while the transfer from the towel to the second mask (wife) was much higher than expected (54% versus 3.05%) (Table 7).

Table 7. Case 3: Expected and observed transfer percentages to each experimental area for each transfer step.

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4. Discussion 

4.1. Case 1 

Observed transfer rates were generally 2–4 times greater than expected for the case 1 scenario. When each of the five replicates was looked at separately, however, several had transfer percentages similar to the expected values. It is possible that if we increase the number of replicates a better measure of variation will be obtained. Additionally, the relative areas of contact on both toys/singlets and the coats, as well as the type of the experimental material used (plastic (toy) and fabric (singlet)), were different from substrates used in the earlier experiments [6], [7] that provided the transfer rate data. These discrepancies may account for at least some of the differences. Total quantities of DNA from skin deposited on fabric (i.e. singlet) were greater than those on plastic (i.e. toy) in this study, similar to previous reports. The larger quantities of DNA deposited on singlets than toys resulted in a greater number of alleles detected on the former. Whilst the main reason for the differences is likely to be due to substrate composition, it may also, in part, be due to the differences in the methods used to collect the DNA from each of the surfaces (i.e. double swabbing versus tape lifting).

In most instances, the donor (perpetrator) profile was the major allele contributor. However, several victim alleles were also found and, perhaps not unexpectedly, most of these were located on the laboratory coats worn by the “victims”. Several victim alleles were also found on the toys and singlets, indicating that not only was donor (perpetrator) DNA transferred from the toys and/or singlets to the laboratory coats, but also that DNA present on the coat (victim) was transferred in the opposite direction; from the coats to the singlets and/or toys.

The most likely explanation for the presence of unknown alleles is that most, or all, of those alleles are part of the “background” DNA present on the coats prior to the experiment [12], [13], [14]. Approximately 20% of the unknown alleles were common to all the coats, and considering that each coat belonged to a different wearer, these alleles have probably come from contact with the toy or doll where one skin donor was used for all deposits. Conversely, comparison of the unknown alleles found on toys and singlets showed that more than 50% were unique to each item and, therefore, are likely to have come from coats during the transfer step, and not from the skin depositor used in the experiments. In the laboratory environment practitioners adhere to strict protocols regarding the use, frequency of change and handling of their individual laboratory coats, all of which are designed to ensure minimal exposure to, and pick up of, exogenous DNA. During the act of criminal activity, where no such measures exist, a significantly greater number of unknown alleles can be expected to be found which may well complicate interpretation.

4.2. Case 2 

The results from the second experiment suggest that, though the postulated explanation of the presence of the DNA at the crime scene is possible, under the set of variables investigated here, transfer of sufficient material to generate a full profile is unlikely. Several variables, including those that have been tested and possibly other, presently unknown ones, could have contributed to the result observed. The handling time selected for handling the trigger hose in this experiment was arbitrary and the actual handling time of the megaphone could have been longer than 1min. Additionally, the use of the megaphone instead of the hose trigger could have introduced extra DNA from saliva during its use. Furthermore the ambient temperature at the crime scene location was considerably higher than that in the laboratory and may well have increased participants’ sweating. All these factors could have resulted in larger quantities of DNA and subsequently more complete profiles from tested objects in the case 2 scenario. It should be noted that although a few alleles were present on tested items in our experiment and assigned to the perpetrator or the victim, such assignment is not infallible, as the overall number of alleles found is very small resulting in low discrimination power and could, in fact, be a result of foreign DNA.

4.3. Case 3 

The third experiment investigated an example of possible multiple DNA transfer events and the results for all transfer steps were greatly different from those expected using the previously described transfer rates. It is possible that the towel used in the experiment (poly/cotton; pile weave) was not collecting and retaining the DNA material as efficiently as the fabric used in Goray et al. [6], [7] (100% cotton; 3:1 Drill weave) thus resulting in lower transfer from the first mask to the towel and, subsequently, greater than expected transfer from the towel to the second mask. Preliminary studies show that a diversity of materials absorb and retain liquids with different degrees of effectiveness [6], [7], [15]. Accordingly, variation in characteristics such as fabric composition (e.g.% of cotton versus polyester), and/or thickness and/or weave of fabric fibres may significantly affect DNA transfer. Similarly, a different type of plastic from the one investigated previously [6], [7] was used and this may explain some of the observed discrepancies. One reason for transfer difference among substrates may lie in their chemical structure [15].

On three occasions in this experiment, positive profiling results were obtained from samples with negative quantitation results. Therefore, one needs to be cautious when making a decision not to proceed with further analysis following negative quantification, especially when trace samples are involved.

When the two gloves used for strangling were analysed separately, the glove from one hand was found to contain far more alleles than the other. This may have been caused by volunteers favouring one hand over the other when “mock strangling” the victim. It could be proposed that the individuals dominant hand is often used more frequently and with greater strength and so will create more friction and lead to the collection of more DNA than the non-dominant hand. Unfortunately, in this experiment, gloves were not labelled whether left or right and, therefore, testing this handedness hypothesis was not possible. Unexpectedly, a small proportion of the observed DNA appeared to be derived from the wearer of the gloves even though only the outside of the glove was sampled. A possible explanation for this could be the manner in which the gloves were handled. The very minor unknown component also found could have been derived from the vector’s hands as it is known that swabs from hands often contain small proportions of DNA from other sources [16].

4.4. General 

The conditions under which these experiments were conducted were not as uniform as those in previous studies on transfer of DNA. The differences between the observed and expected transfer rates could, in part, reflect the more realistic crime scene scenario conditions in the present set of experiments. For instance, the area of deposit and corresponding contact substrate were less defined and the pressure applied was not as precise as in the controlled experiments. Further, new and sterilized items were used in our study whilst casework exhibits would likely contain background levels of DNA acquired prior to the proposed events, thus complicating scenario re-enactment, DNA interpretation and police investigation.

In one of the court cases, a private forensic laboratory was engaged by the defendant to replicate the crime scene scenario proposed by the defence [5]. However, the proposed conditions were replicated only partially, limiting the accuracy and therefore the value of the results obtained. In the future, if such a re-enactment path is to be followed, all known variables should be incorporated, into the experimental design.

If an approach of case scenario reconstruction was utilised during forensic case investigation, the parameters of the experiments would have to be strictly defined in order to align with the proposed sequence of events. Variables, such as type of substrate, duration and type of contact, environmental factors like humidity and temperature, should all be known as fully as possible and incorporated in test scenarios. Where knowledge of events and variables, such as manner of contact and freshness of deposit among others, is unknown and assessed to have a possible significant effect on the ultimate outcome, then any test scenario should incorporate a wide variety of options for that particular variable within the scenario being recreated. Such re-enactment only becomes relevant and possible when clear and specific secondary or further transfer events are proposed during crime investigations or court proceedings and are subject to police due diligence when collecting data and evidence associated with the crime. Whilst it is preferable to re-enact the proposed crime scene scenario(s) encompassing all the known variables, such re-enactment(s) may require resources at a level which may limit it ever becoming common practice in forensic laboratories. Therefore, further research and acquisition of data are needed to create a basic DNA transfer prediction framework. Such a framework or database could be updated with outcomes of future research activities investigating additional variables, repeating and further investigating variables already identified, all of which will increase accuracy and thus confidence in its use. Nonetheless, even with improvements, these data are likely to be less reliable than individual re-enactment information and thus in serious cases where any proposed transfer seems likely after the application of a prediction framework, re-enactment might be the appropriate subsequent step in the investigation. With knowledge that DNA transfer is increasingly an issue in court, it is becoming reasonable to expect an opinion on transfer likelihood from DNA expert witnesses, making further research into this issue of paramount importance.

In conclusion, in an attempt to evaluate the application of DNA transfer percentages generated in previous studies and better understand the nature and effects of different variables on the transfer rates, a study mimicking criminal DNA transfer scenarios was conducted which gave mixed results. Some general trends in line with previous results could be identified; however, some differences between the expected and actual transfer rates were apparent. It is likely that a number of variables affecting transfer rates in the re-enactment scenarios are greater than the number taken into an account when calculating the expected transfer rates under strictly controlled conditions. Thus, additional transfer parameters need to be identified and investigated in order to build a model that can be used by scientists for estimation purposes. Nonetheless, this study shows that in mimicking the described conditions DNA transfer does occur validating the need for further investigations.

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References 

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