Prevalence of DNA from the driver, passengers and others within a car of an exclusive driver

Highlights

• Useful DNA profiles can be obtained from several sites in cars not usually targeted.

• The driver was the major contributor on most sites on the driver side.

• DNA of non-driving co-resident partners was observed on the driver’s side.

• DNA of earlier passengers persisted on car sites after use by others.

• The last passenger was not always the major contributor on the passenger side.

Abstract

Cars are often sampled for DNA to help identify occupants and their possible location(s) within the car. While DNA from the frequent driver is likely to accumulate over time, DNA from previous and/or subsequent occupants, and those whose DNA has inadvertently been transferred to the car, may also contribute to any samples collected. This study investigates how much DNA resides on various sites within cars, and who might contribute to these samples.

A total of 35–36 sites, internal and external, were targeted within four cars with sole long-term drivers. In addition to the car keys, sample sites included the exterior and interior door handles (driver and passenger sides), through to the internal compartments (driver side, middle area and front passengers’ side). Reference samples were collected from the exclusive drivers, their co-resident partners and, where possible, recent passengers.

The driver was always observed as a contributor in DNA profiles from the driver’s side and, in most instances, was the sole, major or majority contributor to the profile. The driver was also observed as a major, majority or minor contributor at several sites on the passenger side. DNA of known recent passengers, close associates of the driver and unknown individuals was collected from many sites on both the driver and passenger sides. These findings may assist in sample targeting within cars and in the evaluation of DNA evidence when propositions relate to the activities performed.

Introduction

DNA profiling of trace biological samples provides forensic scientists with a useful tool to assist with the identification of individuals during investigations of criminal activities. To make good use of this tool it is helpful for those collecting samples to have an awareness of which areas to target in order to collect ample DNA to obtain a profile that is of sufficient quality for interpretation. More importantly, this awareness regarding sample collection will aid in generating a probative profile of the person of interest (POI), if present, preferably as a sole or major contributor.

Cars are frequently involved in criminal activities and sampled for DNA. The types of cases and queries can be diverse, but some of the main reasons for sampling include addressing questions such as ‘who drove the car?’, ‘who was in the car?’, ‘where were they in the car?’ and, ‘how long can DNA persist in a car?’. The areas targeted are dependent on the query and are often kept to a minimum for efficiency purposes. Apart from obvious blood or semen stains on surfaces of a car, or sampling of personal objects left in a car, the main source of biological material sampled is that left by occupants through touching surfaces. This includes, for example, touching surfaces directly with their bare hands, sitting in a seat, or leaning their head against a head rest. The area within a car most targeted for sampling is the steering wheel; other items occasionally sampled include door handles, gear lever and handbrake [[1], [2], [3], [4]].

Application of more discriminating and sensitive STR-multiplex kits has, in addition to expanding our ability to generate profiles from a wide range of items/surfaces [5,6], increased the proportion of samples from touched objects that provide mixture profiles. Many samples collected from cars provide mixed profiles [7,8]. It is desirable to have an awareness of whose DNA is contributing to mixed profiles, and in what proportions, given the history of the surfaces sampled and the individuals interacting with them. As there is an increasing need to also address questions relating to ‘how the DNA got to be on the surface it was collected from’, such awareness may assist evaluation of alternate scenarios provided by prosecution and defence when propositions relate to activities [9,10].

Given that many samples collected from cars are from surfaces that have been touched, coupled with the knowledge that hands readily transfer not just their own DNA but that acquired from contacting other individuals and objects/surfaces contacted by others [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]], one could expect to find DNA from individuals who have never been in the car. It is useful to have an awareness of where DNA of a sole regular driver, their passengers and their close associates may be located within a car. Knowledge of the persistence of DNA derived from occupants of a car after known use and time can also be useful in assisting sample targeting.

There are, however, very few studies published on the success rates of samples collected from the many possible target areas within a car to verify whether the steering wheel or other areas occasionally (or never) targeted may be better, or equally useful, to target depending on the case at hand. Those studies available employed less sensitive profiling kits than those currently commonly used; sampled a few areas other than steering wheels; did not distinguish the various items collected from a car; provided very limited or no information on the types of profiles retrieved, the number of contributors, or their probative value; did not provide any information on the identity of profile contributors other than the POI; concentrated only on the driver’s side; and/or provided very limited information or no information regarding the history of car use [[1], [2], [3], [4],7,8,30]. Whilst there have been a few studies on transfer and persistence of DNA deposited by touching actions [19,20,[22], [23], [24],26,29], none have focused on aspects associated with cars.

Here we report findings of a study analysing DNA samples (using a current commonly used profiling methodology) collected from 35–36 areas within each of four cars, including the exterior and interior car door handles (driver and passenger side) through to the internal compartments (driver’s side, middle area and front passenger’s side), and car keys. Reference samples were also collected from the exclusive drivers, their co-resident partners and recent passengers. The cars were selected as having an exclusive owner with a known history of use and occupancy. While some results pertaining to the steering wheel of these cars have been reported previously [30], here we interpret the results of this item within the broader context of all items sampled and provide further background information relating to this item. The profiles, along with the known histories of the cars (acquired using an extensive questionnaire), were evaluated to broaden our understanding of DNA transfer, prevalence, persistence, and recovery (TPPR) related issues, as well as to assist criminal investigations involving cars