Abstract
Advances in DNA typing methods have enabled forensic scientists to obtain genetic profiles from cells shed onto objects that have been touched or handled. This type of DNA is called “contact DNA,” and crime labs are currently receiving many requests from attorneys and investigators to try to find it on crime scene evidence and analyze it. Cartridge cases and firearms are common types of evidence recovered from crime scenes and are potential sources of contact DNA because criminals may handle cartridges or gun grips directly, without gloves. As a result, analysts are often asked to swab cartridge cases and gun grips for contact DNA when examining evidence from crime scenes where firearms were used. DNA typing is expensive and time consuming, and many crime labs are backlogged with unsolved cases. Moreover, many analysts report that, in their experience, fired cartridge cases are a very poor source of DNA and that it is probably a waste of time and money to test them. Therefore, a comprehensive study of the factors that affect the deposition of contact DNA on ammunition and guns, and the types of cartridges and gun grips that are most likely to generate useful genetic information, is badly needed. This project examined whether it is possible to generate human DNA profiles from fired and unfired cartridge cases, and from gun grips, under varying conditions relevant to forensic casework Seven common cartridge cases were examined, including 0.22 caliber brass cases, 0.38 caliber brass, aluminum and nickel cases; and 9mm brass, steel, aluminum and nickel cases. Gun grips made of wood, plastic and rubber, with either textured or smooth surfaces, were also sampled. In addition, two other variables were studied: (1) whether the length of time between hand washing and handling affects the amount of DNA shed onto a case or grip, and (2) whether the amount of time the person handles a case or grip affects DNA shedding. The Applied Biosystems AmpFℓSTR Minifiler™ system was used to genotype all of the samples. While it is less informative than the more widely used AmpFℓSTR Identifiler™ system, it is more sensitive. However, samples producing Minifiler™ profiles with 6 or more alleles and containing at least 0.1 ng of DNA were also genotyped with Identifiler™ to see if more genetic information could be obtained. A total of 690 swabs were collected and analyzed, 590 for the cartridge case studies and 150 for the gun grips studies. Of these, 51 samples were contaminated with DNA from the examiner or another source and were excluded from downstream analyses. Therefore, results are reported for only 501 cartridge cases and 138 gun grips. The DNA from the samples was extracted on the BioRobot® EZ1 using the trace protocol, amplified with ABI AmpFℓSTR Minifiler™ kits, and genotyped on a 3130 Prism® xl genetic analyzer. Only profiles containing alleles above a minimum threshold of 75 RFUs were included in the evaluation of a profile. Samples with alleles present at all loci were designated as “full profiles” since, during casework involving an unknown donor, they could be interpreted by an analyst as containing all of the donor’s alleles. Samples with at least one allele observed at any of the loci were designated as having “partial profiles,” and samples with no alleles were designated as having “no profile.” Overall, 135 of the 639 non-contaminated samples yielded “full” or “partial” profiles with Minifiler™, 63 from the cartridge cases and 72 from the gun grips. Of the samples that also underwent additional AmpFℓSTR Identifiler™ typing, 24 produced profiles. One “full profile” was produced from a cartridge case and six “full profiles” were produced from gun grips. In addition, four “partial profiles” were produced from the cartridge cases and 14 were produced from the gun grips. The caliber of the cartridge, the material from which it was made, and the length of time it was handled did not affect DNA recovery. Similarly, the material and texture of the gun grip and the length of time it was handled did not alter DNA yields. However, cartridge cases and grips that were handled one hour after donors washed their hands yielded significantly more DNA than cases and grips handled immediately after hand washing. In addition, unfired cases yielded significantly more DNA than fired cases, and gun grips yielded significantly more DNA than cartridge cases. After the genotyping was complete, all “full profiles” and “partial profiles” were compared to the donor’s known profile to detect allelic drop out. While partial allelic drop out is common in contact DNA profiles analyzed by Identifiler™, the second allele can usually be detected on the electropherogram below the stochastic threshold but above background. Therefore, the second allele can be used for inclusionary or exclusionary purposes. However, 58 of the 135 Minifiler™ profiles exhibited the complete drop out of an allele at one or more heterozygous loci. This presents a challenge because, if a contact DNA sample is from an unknown source, its Minifiler™ profile could be interpreted incorrectly and potentially mislead an investigation. The fact that 7.2% of the Minifiler™ profiles in this study exhibited contamination only serves to exacerbate the problem. Therefore, although Minifiler™ has the advantage of being more sensitive than Identifiler™, analysts should carefully weigh the need for sensitivity against the possibility of increased contamination and the complete drop out of some alleles. In some instances, Minifiler™ may be the best choice for obtaining genetic information from a contact DNA sample. However, it is critical that forensic DNA analysts understand the limitations of the system and interpret their results with caution. This study provides a wealth of information about the factors that may influence DNA recovery from cartridge cases and gun grips and should provide the forensics community with empirical support for accepting or denying requests for contact DNA analyses. In particular, this study indicates that trying to obtain genetic information from fired cartridge cases is probably not worth the time and expense involved. Investigations would be better served by developing new methods for generating such profiles or by focusing on evidence that is likely to be more probative.