Chapter 4: Sources of Biological Evidence

Beyond traditional cellular sources, the chapter explores nucleic acid markers that provide additional investigative value: messenger RNA enables tissue-specific identification by revealing which genes are actively expressed in particular biological sources, while microRNA—small regulatory molecules processed through enzymatic pathways involving Drosha and Dicer enzymes—offers supplementary profiling opportunities in degraded or limited samples. The chapter then systematically addresses major tissue sources encountered in casework. Skin represents a critical source of trace DNA, particularly in touch evidence where minimal cellular material transfers between individuals during contact. Hair analysis depends heavily on the hair growth cycle, with anagen phase hairs containing actively dividing root cells that yield superior DNA, while telogen phase hairs shed from the scalp may provide degraded or absent nuclear material. Bone tissue, despite its mineral composition, harbors multiple cell types including osteocytes embedded within the matrix and osteoclasts that participate in bone remodeling, making skeletal remains viable for DNA recovery even in advanced decomposition. Dental tissue, particularly the pulp chamber protected within tooth structure, preserves biological material exceptionally well due to its isolated anatomical position, making teeth among the most reliable sources for extracting DNA from severely compromised remains. Together, these diverse biological sources provide forensic scientists with multiple avenues for establishing identification and connecting individuals to crime scenes.