Species identification in mammals from mixed biological samples based on mitochondrial DNA control region length polymorphism.

The ability to identify the species origin of an unknown biological sample is relevant in the fields of human and wildlife forensics. However, the detection of several species mixed in the same sample still remains a challenge. We developed and tested a new approach for mammal DNA identification in mixtures of two or three species, based on the analysis of mitochondrial DNA control region interspecific length polymorphism followed by direct sequencing. Contrary to other published methods dealing with species mixtures, our protocol requires a single universal primer pair and is not based on a pre-defined panel of species. Amplicons can be separated either on agarose gels or using CE. The advantages and limitations of the assay are discussed under different conditions, such as variable template concentration, amplicon sizes and size difference among the amplicons present in the mixture. For the first time, this protocol provides a simple, reliable and flexible method for simultaneous identification of multiple mammalian species from mixtures, without any prior knowledge of the species involved.

Helping to distinguish primary from secondary transfer events for trace DNA.

DNA is routinely recovered in criminal investigations. The sensitivity of laboratory equipment and DNA profiling kits means that it is possible to generate DNA profiles from very small amounts of cellular material. As a consequence, it has been shown that DNA we detect may not have arisen from a direct contact with an item, but rather through one or more intermediaries. Naturally the questions arising in court, particularly when considering trace DNA, are of how DNA may have come to be on an item. While scientists cannot directly answer this question, forensic biological results can help in discriminating between alleged activities. Much experimental research has been published showing the transfer and persistence of DNA under varying conditions, but as of yet the results of these studies have not been combined to deal with broad questions about transfer mechanisms. In this work we use published data and Bayesian networks to develop a statistical logical framework by which questions of transfer mechanism can be approached probabilistically. We also identify a number of areas where further work could be carried out in order to improve our knowledge base when helping to address questions about transfer mechanisms. Finally, we apply the constructed Bayesian network to ground truth known data to determine if, with current knowledge, there is any power in DNA quantities to distinguish primary and secondary transfer events.

Macroscopic observation of the morphological characteristics of the ammunition gunpowder.

At the present time, optical examinations are not used in routine to analyze the gunshot residues, except for the counting and the localization of particles. Indeed, the sequence of examinations often starts immediately with destructive techniques, contrary to the widely accepted law saying that we should progress from general to particular and from non-destructive to destructive and which is recommended for all the fields of forensic sciences. When a cartridge is shot, and before the projectile leaves the gun, all the powder grains should must be completely burnt; however, if this ideal case does not happen, it is possible to find unburnt and/or partially burnt gunpowder particles [H.H. Meng, R. Caddy, Gunshot residue analysis-a review, J. Forensic Sci. 42 (1997) 553-570]. The goal of this paper is to study the morphological characteristics of the powder before and after the shot, to establish if it is possible to determine which type of ammunition has been used on the basis of these characteristics. A set of 181 cartridges of different calibers was considered and various tests carried out to evaluate the technique. On the basis of the observation of the gunpowder particles found on the target, a list of potential cartridges can be established with the actually shot cartridge always found among them. It is important to underline that a maximum of eight cartridges were proposed for each experiment. Consequently, the method can be judged very discriminating. A database was then created, including information related to the morphological characteristics of the gunpowder before and after the shot as well as to the class characteristics of the analysed cartridges.