What is Forensic Biology?
Forensic biology is applied in any cases where the analysis of biological material may aid an investigation, for example crimes such as murder or rape. The main biological materials of interest are blood, semen, other body fluids (such as saliva), hairs and fibres.
It is useful to read about DNA as this is also a form of biological analysis. A DNA sample can be taken from most biological material and a DNA profile produced. DNA profiling is a tool used by biologists to assist in the interpretation of scientific evidence. A person's DNA profile can be compared against that of any recovered biological material to determine whether an individual may have been present at a crime scene.
Blood and Blood Pattern Analysis
In a physical assault where the victim sustains bleeding injury, blood may be transferred to the clothing of the assailant, on to weapons used in the assault, or on to other surfaces at the crime scene.
The finding of such transferred blood stains, even though blood stains may be minute or attempts may have been made to wash blood away, can therefore be critical in demonstrating a link between a suspect and a crime, or in establishing that a weapon, vehicle or address is implicated in an offence.
Methods for searching for such blood traces range from painstaking microscopic examination of clothing and weapons through to the use of specialist alternative light sources and chemical screening methods that can detect blood stains invisible to the naked eye. In many instances however, the form and pattern of the transferred blood staining can also provide vital information about the mechanisms that led to its deposition.
Blood pattern analysis is the study of the characteristic patterns of blood staining formed by different activities, and its application at the crime scene, or in the laboratory when examining clothing and weapons, can help reconstruct the sequence of events that resulted in the blood transfer. For example, the patterns of blood spatter may indicate where at the crime scene the victim was struck, it may show that they were assaulted while standing up or on the ground, it may indicate that a weapon is likely to have been used to strike the victim, and it may highlight that the assailant is likely to be heavily covered with blood spatter as a result of the assault.
The examination of textiles for damage can play a significant part in the examination of clothing in physical and sexual assaults. The aim of such examinations is to establish if any damage observed could be related to the incident and to provide information that may assist in determining particular versions of events.
Forensic scientists may be able to provide opinions on;
- The form of the damage observed e.g. a cut in fabric vs a tear or simply due to wear
- If the damage has been affect by repeated washing or wear, which may assist in an opinion on the age of the damage in broad terms
- The blade type / broad dimensions of a knife used in an assault
- The type of implement and action that caused the damage e.g. a knife in a stabbing motion or a slash with broken glass
The scientist may choose to recreate any damage seen in order to understand the mechanism and force required to create the damage observed.
Textile Fibre Analysis
During the commission of a crime, fibres may transfer between the clothing of the offender and of the victim, or between the clothing of the offender and the crime scene, and if these fibres persist on the recipient surface they can be recovered by the crime scene investigator or scientist. Recovered fibres can be compared with the possible donor fabric to establish if physical contact might have occurred.
Fibre evidence depends upon the donor fabrics shedding their fibres in appreciable quantities and the nature of the offence providing opportunity for those fibres to be transferred. For example, a snatch of a handbag in the street may involve little or no clothing-to-clothing contact, whereas a struggle as a victim grapples with a rapist may provide ample opportunity for fibre transfer. In addition, the recipient surface must be such that fibres from the donor fabric will adhere when contact occurs and persist for a period of time.
Fibres will gradually be lost from the recipient surfaces if the clothing is worn or if the object is outdoors and exposed to inclement weather conditions, for example. Fibre persistence will vary depending on the type of transferred fibres and the texture of the recipient fabric, so although fibre transfer may have occurred, these fibres may not persist long enough to be recovered for examination.
Fibres thought to have been transferred during a crime are examined closely by high power microscopy to identify fibre type and morphological features. They may then be compared with the fibres comprising possible donor fabrics by high power microscopy, instrumental colour analysis and chemical composition.
Just as the forensic value of textile fibres lies in their potential to be readily shed and transferred during physical contact, so the capacity for hairs to be lost from the skin during the normal growth cycle and transferred to other surfaces makes them a potentially valuable source of evidence.
The microscopy of hairs identifies key morphological features that can help establish if the hairs are human or animal, and in the case of human hair it can establish the donor’s ethnic origin, what part of the body the hairs came from and how a hair became damaged. In the case of animal hair, microscopy is one of the main techniques for establishing the species of origin, for example in poaching and other wildlife crime cases. As with textile fibres, comparison microscopy can be used to compare a known reference sample with the recovered sample to establish if the recovered hairs could have come from a specific person or animal.
Whilst nuclear DNA analysis of hair roots and mitochondrial DNA analysis of hair shafts can now be used to identify the possible donors of hairs, microscopic examination of hairs still plays an important role in selecting suitable hairs for DNA analysis, and in answering questions that DNA analysis cannot, such as where on the body a recovered human hair originated.
Semen is the most widely encountered body fluid in sexual offence examinations. Through chemical testing and microscopy, Forensic scientists can identify the presence semen on intimate swabs or items such as clothing, bedding and condoms. DNA profiling, including Y-STR profiling is then utilised in an attempt to identify the source of the semen.
As well as establishing the presence of semen on intimate swabs, the amount and distribution of semen detected on swabs will be assessed. This can allow Forensic Scientists to offer an opinion on a likely time frame in which sexual intercourse occurred.
In addition, distribution of semen on clothing items, for example, underwear may also assist with an interpretation supporting or negating versions of events.
Other body fluids including blood and saliva are also commonly encountered in the examination of items for sexual offences.
A robbery was carried out at a pub in the West End of Edinburgh. As one of the robbers was leaving he spat on the pavement outside the pub. Saliva was recovered and a DNA profile obtained. The two robbers escaped in a stolen vehicle which was subsequently recovered. When the owner came to collect his car he complained about all the tissues on the floor of the car. The second robber had a cold and discarded his used tissues in the car. A DNA profile was obtained from the nasal secretions on the tissues.
Two suspects were later arrested and DNA samples taken from each. These matched the profiles obtained from the saliva and nasal secretions and the two robbers were successfully prosecuted.