Serology is the detection, identification, and typing of body tissues, either in native form or as stains or residues left at a crime scene. Most often, the tissue of interest is a body fluid such as blood or semen; however other tissues such as hair or bone are encountered. The detection and identification process begins with a physical examination, followed by a screening test, and may include confirmatory identification and sample typing. Typing is the detection of genetically determined characters in the sample. A difference in type between the stain and reference sample from the postulated source may provide a positive elimination. If there is no elimination, the results can be evaluated against population frequency data to give a measure of the significance of the failure to eliminate.
The features of a good typing system are that it
- shows variability from person to person but is constant within one individual,
- is stable in shed form,
- can be detected reliably at the concentrations found in forensic samples,
- has a known and stable frequency of occurrence within the population.
It is not required that a typing system be based on an inherited characteristic, although in practice they all are.
The very presence of biological fluids, specifically blood and semen, may be an indication of a serious crime and of evidential value. The physical distribution of blood stains at the scene or on clothing may produce valuable information about the crime.
Characterization of biological fluids is often used to associate forensic evidence with an individual. Testing allows conclusions to be drawn as to the person from whom the fluid originated.
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