DNA Analysis and Allelic Comparison of Biological Samples at the Crime Scene: A Case Report

Abstract

Expert work at crime scenes can identify and collecting such traces, enabling the determination of authorship using molecular markers. The objective of this work is to report a case that shows the viability and efficiency of forensic work at the crime scene and in the Forensic Biology Laboratory, together with the police investigation. This is a sexual crime involving two male individuals. At the scene of the crime the following biological samples were found: the victim's oral swab (1); nail bed from victim's fake nails (2); used condom, close to the victim, external (3A) and internal face (3B); a used condom, inside the toilet, external face (4A), and internal face (4B); biological sample on the bed sheet (5). The samples were subjected extraction and amplification and to capillary electrophoresis. The results indicated that samples 2 and 3A had a male genetic profile, with correspondence to sample 1 (victim's reference).  After six months, with the investigation still ongoing, a sample of oral swabs was obtained from a suspect (6) (reference suspect). In sample 4B, a mixture of genetic material from two people was obtained, corresponding to questioned samples 1 and 6. Two important points stand out for the successful outcome for objective evidence: The first was adequate collection and preservation of biological traces found in crime scenes by the scene expert, allowing the adequate products from DNA amplification in laboratory procedures developed by the laboratory expert; the second refers to the integration between investigative and forensic activities during police work.