Biological sources of DNA: The target materials for forensic DNA typing

Abstract

Among physical evidences encountered at the crime scene, biological evidences, viz., blood, semen, vaginal secretion, saliva, urine, and sweat, are the most ubiquitous in nature, and their presence aids in linking perpetrator to the victim as well as crime scene. Recognition of biological fluids as substantive evidences is endorsed by presence of DNA in them. Advancing forensic DNA typing techniques have a great potential in characterization and individualization of biological evidences encountered during criminal investigation, but their applicability on each biological fluid for human identification varies in a great deal due to variation in the amount of nucleic acid available within the fluid. DNA concentration is relatively high in fluids such as blood that contains large number of cells while fluids such as urine and sweat possess low amount of DNA. Besides reference biological samples, forensic DNA typing can be implemented for detecting presence of traces of biological fluids on physical surfaces. The concept of "Touch DNA" or "Transfer DNA" involves analysis of low amount of DNA deposited on the surfaces that have come in human contact and can constructively help in associating evidences with perpetrator and/or victim. However, identification of sources of such materials, their collection methods, and preservation conditions can influence the quality of result. Further to this, degradation, impurity, contamination, and presence of inhibitors in such evidences demand purification and isolation of high-quality DNA. This chapter deals with various biological sources of DNA commonly encountered at the scene of crime and their evidential value, along with various factors and conditions affecting forensic DNA typing of such samples. This chapter deals with various biological samples used for forensic DNA typing along with various factors and conditions that affect forensic DNA typing.