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 [1]. 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 (Figure 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 importance 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, deposited, 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 [2]. Hairs are naturally shed, and can also be pulled out through physical contact and can be recovered from crime scenes. Naturally shed hairs tend to have

Table 3.1 Types of biological material that can be recovered from a crime scene. The DNA profiles generated from crime scene material are compared against reference profiles that are provided by suspects, and in some cases, the victims

Scenes of Crime Reference Samples

Blood Blood

Semen Buccal swabs

Hair Pulled hairs (containing roots)

Epithelial cells - shed skin cells: Saliva Dandruff Clothing Cigarette butts Drinking vessels/food Urine Vomit Faeces Touch DNA

very little follicle attached and are not a good source of DNA, whereas plucked hairs or hairs removed due to a physical action often have the root attached, which is a rich source of cellular material.

The four most common nucleated cell types that are recovered from scenes of crime are white blood cells, spermatozoa, epithelial cells and hair follicles (Figure 3.2).

Figure 3.1 Blood is the most common form of biological material that is recovered from crime scenes. (a) Large volumes of blood can be collected using a swab, if the blood is liquid then a syringe or pipette can be used (picture provided by Allan Scott, University of Central Lancashire) (b) Blood on clothing is normally collected by swabbing, or cutting out the stain (picture provided by Elizabeth Wilson) (see plate section for full-colour version of this figure)

Figure 3.1 Blood is the most common form of biological material that is recovered from crime scenes. (a) Large volumes of blood can be collected using a swab, if the blood is liquid then a syringe or pipette can be used (picture provided by Allan Scott, University of Central Lancashire) (b) Blood on clothing is normally collected by swabbing, or cutting out the stain (picture provided by Elizabeth Wilson) (see plate section for full-colour version of this figure)

Figure 3.2 Common cell types that are recovered from scenes of crime: (a) white blood cells; (b) spermatozoa; (c) epithelial cells (from saliva); (d) a hair shaft with the follicle attached (the cells have been stained with haematoxylin and eosin)

Figure 3.2 Common cell types that are recovered from scenes of crime: (a) white blood cells; (b) spermatozoa; (c) epithelial cells (from saliva); (d) a hair shaft with the follicle attached (the cells have been stained with haematoxylin and eosin)

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