3 Biological material – collection, characterization and storage
The sensitivity and evidential power of DNA profiling have impacted on the way in which crime scenes are investigated. Because only a few cells are required for DNA profiling, crime scene examiners now have a much wider range of biological evidence to collect and also have a much greater chance of contaminating the scene with their own DNA.
Sources of biological evidence
The human body is composed of trillions of cells and most of these contain a nucleus, red blood cells being a notable exception. A wide variety of cellular material can be recovered from crime scenes (Table 3.1). Each nucleated cell contains two copies of an individual’s genome and can be used, in theory, to generate a DNA profile under optimal conditions . In practice, 15 or more cells are required to generate consistently good quality DNA profile from fresh material. Forensic samples usually show some level of degradation and with higher levels of degradation, more cellular material is required to produce a DNA profile. If the material is very highly degraded then, even with the high sensitivity of DNA profiling, it may not be possible to generate a DNA profile. The biological material encountered most often at scenes of crime is blood (Fig- ure 3.1). This is mainly because of the violent nature of many crimes and also because it is easier to visualize than other biological fluids such as saliva. Other frequently encountered samples include seminal fluid, which is of prime im- portance in sexual assault cases; saliva that may be found on items held in the mouth, such as cigarette butts and drinking vessels, or on bite marks; and epithelial cells, de- posited, for example, as dandruff and in faeces. With the increase in the sensitivity of DNA profiling the recovery of DNA from epithelial cells shed on touching has also become possible . Hairs are naturally shed, and can also be pulled out through phys- ical contact and can be recovered from crime scenes. Naturally shed hairs tend to have
An Introduction to Forensic Genetics W. Goodwin, A. Linacre and S. Hadi
© 2007 John Wiley & Sons, Ltd