Developing Analyses of Biological Evidence: Predicting Eye Color, Determining the Source of Bodily Fluids, and Locating Trace Evidence Within Guns

Several studies reported in the January 2013 Issue of Forensic Science International: Genetics are of interest for future developments in forensic evidence.

Research is being done on predicting eye color, hair color, and skin color from DNA. Currently, 37 gene sequences (SNPs or single nucleotide polymorphisms) have been identified as playing a role in these traits, and, of these, six have previously been used for predicting eye color. The current study tested additional gene sequences, beyond the previously used six SNPs, in order to test the ability to keep the cost of the test in balance with any gains in predictive value and reliability. It is important to note that these tests assign likelihoods of particular eye colors of the source person and do not provide an absolute determination of that eye color. Such tests may be used in the future to help develop possible suspect profiles.

Another type of genetic material, mRNA (messenger RNA, the “template” copy of DNA coding used for assembling specific proteins within a cell), has been found useful in determining the source of biological fluids. A study of mRNA markers identified specific markers for blood, saliva, semen, and vaginal secretions. The ability to identify the tissue source of a DNA profile would have important forensic uses because currently it is not possible to determine whether the DNA profile developed from a particular item of evidence comes from, for example, a blood droplet or from skin cells previously left on the surface of that item of evidence.

Finally, a study examined the firearms used in 20 cases of homicide or suicide in which there was a close-contact shot fired. The inside of the barrels of the guns were sterilely swabbed from the front of the weapon (making sure to avoid contact with the muzzle) in each case and also swabbed from the rear of the weapon in 16 of the cases. Usable biologic material for DNA analysis was found in 17 of the guns. Furthermore, after the initial samples were collected, each gun was fired one additional time, and usable genetic material was still found in 14 of those guns. Not only may testing of genetic material from within gun barrels be used in crime scene investigation in the future, but parameters may be developed for specific gun types such that, if biological evidence is found within the barrel of a gun, investigators will be able to determine if the firearm likely was fired from within a certain distance from the victim’s body.

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