Forensic biology is the application of biological analysis methods, particularly serological methods, to legal investigations. Serology involves the investigation of bodily fluids, particularly the likes of blood, semen, saliva, all of which are commonly found at certain crime scenes.
There are numerous types of bodily fluid that may be found at a crime scene or on a victim, all of which have the potential to be analysed and used in the identification and incrimination of the perpetrator. The examination of such substances can not only provide clues as to the identity of the offender, but also help investigators develop a detailed picture of the sequence of events which occurred. The presence of certain bodily fluids can be excellent indicators of what has occurred. For example, the presence of semen may suggest a recent sexual encounter, whether consensual or otherwise. Perhaps more obviously, blood at a crime scene is often indicative of some form of physical struggle, assault, or even murder. The analysis of bodily fluids may also determine the presence of quantities of certain substances in the body, such as alcohol or toxins. More information on this is available on the Forensic Toxicology page.
Bodily fluids can be divided into two categories: excreted fluids and secreted fluids. Excreted fluids that may be found at a crime scene include faeces, vomit, bile, and sebum (skin oil). Secreted fluids include blood, plasma, semen, saliva, female ejaculate, and urine.
When a potential bodily fluid is first discovered at a crime scene, actions may be required to visualise the stain. Some biological samples are difficult to see with the naked eye, and require particular light or chemical additions to reveal their presence. Presumptive tests may be conducted to give some indication as to the identity of the substance, though these tests are by no means conclusive, and further analysis will be essential. The sample must be then collected and stored appropriately so as to preserve its integrity as best as possible. Wet samples will often be swabbed, with the swab then being placed in a vial or other airtight container. Individual samples should obviously all be stored separately to prevent contamination. All biological samples are generally dried or frozen during transport and storage. If the samples are to be dried, they should be left to dry by air without the addition of heat, as heat can be damaging to such specimens. These extensive measures are taken to not only protect the samples for analysis, but also protect the staff handling the samples from biohazards, such as infection from a biological sample. The sample will then be transported to a laboratory so that the analysis can be conducted.
The primary goal of this analysis will be to establish exactly what the sample is. Though the answer may seem obvious from the appearance of the sample, conclusive tests should always be conducted. Specific tests will be discussed in more detail later on. The substance should also be subjected to species-specific tests, as the biological sample may belong to another animal rather than a human. After the completion of such confirmatory tests, DNA analysis may be conducted to attempt to identify the secretor of the sample.
A biological sample may not always contain sufficient DNA to obtain a DNA profile. Individuals may be known as secretors or non-secretors. Secretors present aspects of their blood’s protein in other bodily fluids, whereas non-secretors will not have sufficient levels of protein in their bodily fluid to establish a match between two samples. Fortunately, the percentage of the population who are non-secretors is comparatively small.
One of the most common types of bodily fluid found at crime scenes, particularly the scenes of violent crimes, is blood.
Though the appearance of blood is often quite distinct, chemical tests are essential to confirm its identity. Initially presumptive tests are used at the scene, which will merely confirm that the substance in question is most likely blood, though the species is not established at this point. Presumptive blood tests are usually based on the colour change or chemiluminescence of a particular reagent when it comes into contact with the haemoglobin in blood.
Luminol is frequently used in initially identifying bloodstains, particularly if the perpetrator has attempted to clean up the blood, thus rendering it invisible to the naked eye. The presence of blood causes chemiluminescence, the emission of light as a result of a particular chemical reaction, in this case of a blue-green colour. However luminol has been known to react with other substances, including bleach, saliva, and various animal and vegetable proteins.
The Kastle-Meyer or phenolphthalein test is another presumptive blood test. The stain in question is collected with a cotton swab before drops of ethanol and phenolphthalein indicator are added. If no colour change occurs, peroxide is then added. This detects the presence of the enzyme peroxidase in the blood, producing a pink colouring if present.
Leucomalachite Green, or LMG, is similar to the Kastle-Meyer test, replacing the phenolphthalein with leucomalachite green. When added to the substance, a green colour will be produced if blood is present.
Hemastix are plastic strips originally used as a form of urine test. In the presence of blood, the strip will take on a green colour. These strips are particularly beneficial in that they are small and easily taken to crime scenes, allowing them to act as a simple, instant presumptive blood test.
It is then necessary to confirm that the blood is of human origin, as animal blood may be completely irrelevant to the crime under investigation. The precipitin test is used to determine the species of the blood’s origin. Blood contains different proteins which vary between species, meaning that the proteins in the blood of one animal may not be accepted by the blood of another species. If a foreign protein is detected, antibodies are produced to protect the body from harm. Serum for this precipitin test is commonly obtained from rabbits, as they have produced antibodies to destroy a small amount of human blood injected into them. This produced anti-human serum is added to the suspected bloodstain. If the blood is of human origin, the serum will precipitate its proteins, which can be visually observed.
Samples of wet bloodstains will usually be collected using a swab, later sealed in an airtight container. Dried bloodstains may be scraped onto a sheet of clean paper or into an appropriate bag. Any bloodstained items that are collected should be stored separately from one another to avoid contamination and damage to the stains.
Blood typing is one form of categorising blood, known as the ABO System. A, B, O and AB are the primary blood groups, based on the presence of certain antigens on the surface of the blood cells. Before DNA testing, blood groups were used as a method of eliminating or incriminating suspects, though obviously not exclusively. Though the use of blood groups cannot specifically identify the individual from whom the blood originated from, they can narrow down the field of search and eliminate particular groups. Rhesus antigens are also commonly studied within the blood typing system. An individual whose cells do possess the Rhesus antigens is known as being RH positive, likewise those without the antigens are known as Rhesus negative. Most people, about 85%, do possess the Rhesus antigens.
The frequencies of certain genes within different blood groups may vary between different races or groups of people, potentially narrowing the field of suspects further. For example, Rhesus antigen V is present in around 40% of West African Negroes, but only 0.5% of Caucasians. The blood type itself may at least suggest the most likely race of the suspect or victim. Type A blood is most common among Europeans, whilst type B is found ore commonly among Africans and Asians. Similarly, type AB is most common amongst those of Japanese origin, and the O type is frequently seen within the Latin American and Native American groups.
There is also a more physical approach to the analysis of blood found at crime scenes. To find out more about this, see the Bloodstain Analysis page.
Saliva is the clear liquid produced in the mouth for various purposes, primarily to act as lubrication for food and provide the enzyme amylase to begin the breakdown of this food. Composed of water, enzymes, various electrolytes, mucus, and epithelial cells from the inside of the cheeks, it is ideal for DNA profiling. As it contains the same proteins as blood and urine, saliva can also be analysed to detect the presence of drugs and toxins.
Collecting a saliva sample from a suspect has become the most common method of collection when carrying out DNA testing and comparison, as it is simpler and far less intrusive than obtaining a blood sample. A cotton swab is rubbed along the inside of the suspect’s cheek, collecting a sample of saliva and epithelial cells. Saliva will also be of great significance if found at crime scenes, such as on the victim of a sexual assault, on the cigarette end, or around the rim of a glass or bottle.
Due to the high levels of amylase present in saliva, testing for this enzyme is a presumptive test for saliva. However this enzyme is also found in lower levels in other bodily fluids. Various chromatography and spectroscopy methods are frequently used in the extensive analysis of such samples.
Commonly found at the scene of a sexual assault or other sexually-motivated crime, semen plays a crucial role in identifying the perpetrator and linking him to the scene. Semen is the fluid expelled during male ejaculation, designed to carry and support spermatozoa, the sperm cells. In a single ejaculate of semen it is estimated that there are on average a quarter of a 250 million sperm cells, making semen ideal for DNA profiling.
In some cases the fluid may be difficult to see with the naked eye. As previously mentioned, certain alternative light sources can often visualise latent evidence, particularly traces of biological fluids. Ultraviolet light causes semen to fluoresce, indicating its location whilst not damaging the evidence itself.
The acid phosphatase test is one of the most common methods used to detect the presence of semen, due to the high levels of the enzyme in human semen. However acid phosphatase is present elsewhere in the body, meaning the test may react positively with other bodily fluids, therefore this should only be used as a presumptive test for semen.
Prostate-Specific Antigen, also known as SPA or p30, is a glycoprotein produced in the prostate gland. This is equally useful in detecting the presence of semen, though is once again only a presumptive test, as PSA is also found in urine.
The microscopic detection of sperm is a more confirmatory method of discovery. Using microscopic equipment, it is often possible to view the sperm cells, proving their presence. However the older a biological sample is, the further decomposition may have progressed, therefore fresh and well-preserved samples are preferred.
For detailed information on DNA and DNA profiling, visit the DNA Analysis page.
Careers & Education
Laboratories and police forces will generally have a forensic biologist, biochemist or other technician on their staff to conduct the analyses of biological evidence amongst other work. Some institutions may have a forensic serologist on their team to specifically focus on the study of body fluids.
Such a role generally requires a Bachelor’s degree in a pure science such as biology or chemistry. Higher positions may necessitate a Master’s degree or even a PhD. The salaries of these roles can vary greatly.