DNA EVIDENCE SAMPLES
DNA analysis has been successfully conducted on a number of biological substances. The image below identifies those biological substances commonly encountered in criminal investigations and is by no means exhaustive. For example, DNA analysis has been successfully conducted on drinking glasses, cutlery, baseball caps, toques, used sanitary napkins, adhesive bandages, pap smears, envelopes, stamps, mouth-guards, partial dentures, wedding rings, shavers, children’s toys, partially eaten food and contact lenses.
As with all physical evidence recovered from crime scenes, the results of subsequent analysis will depend on the quality and quantity of the evidence recovered. Investigators should work closely with forensic identification personnel at the crime scene to ensure appropriate samples are obtained. Similarly, established procedures should be followed when obtaining biological samples from human subjects.
Blood is the most significant biological substance in forensic DNA investigations. It is the most common substance found at serious crime scenes and is the preferred substance for analysis purposes. Investigators are cautioned to exercise care and not assume blood located at a crime scene necessarily originates from a common source (such as the victim). Depending on the facts of a particular incident, forensic identification technicians should collect representative samples of blood and other biological samples from all crime scene locations. Similarly, incidental to lawful arrest, a suspect's clothing should
be seized and analyzed for its potential evidentiary value (e.g., blood or semen stains).
Semen/ Sperm/ Vaginal Secretions
Sperm is a valuable source of DNA for analysis purposes and like blood, a common biological substance recovered from crime scenes. On average, males produce between fifteen to thirty millilitres, or approximately one to two tablespoons, of ejaculate with each ejaculation. Distinction must be made between sperm and the seminal fluid in which sperm is contained. Seminal fluid itself does not contain DNA, and a vasectomized male's ejaculate will not contain sperm. Certain rare medical conditions may also render males aspermic. DNA analysis of seminal fluid can still be successful however as epithelial cells suitable for analysis may be sloughed from the urethra during ejaculation.
Several studies have examined sperm survival in the reproductive tract of both living and dead females.1s In living victims of sexual assault, spermatozoa have been shown to survive for up to six days in the vagina, seventeen days in the cervix, thirteen hours in the mouth, sixty-five hours in rectal specimens and forty six hours on anal swabs. Spermatozoa have also been shown to survive from two weeks to four months in vaginal specimens taken post-mortem.
Certain investigations may require determining whether vaginal secretions are present on objects alleged to have been inserted during sexual assaults. In these circumstances, despite the absence of blood, cells may be sloughed from the lining of the vagina and contained in the secretions. Analysis can then be performed on the cells in the secretion recovered from an object to determine if the alleged item was inserted.
Body tissues such as muscle and flesh may be suitable for DNA analysis. Living skin is a particularly rich source of DNA, but non-living tissue can also be used (e.g., tissue recovered from a suspect vehicle in a pedestrian hit-and-run or fatal investigation) dependant on exposure to environmental conditions.
Bone/ Bone Marrow
Bones from skeletonized human remains are frequently submitted for analysis in an effort to assist in determining identity. Skeletonized remains may yield useable DNA long after death has occurred. Depending on the age of the remains, analysis will be conducted on the actual bone, as any bone marrow will have degraded. Figure 5 illustrates a section of bone removed from a femur for the purpose of DNA analysis. Note the amount of bone removed does not significantly alter the structure of the bone itself.
In missing person investigations in which foul play is strongly suspected, consideration should be given at the outset, to obtaining personal belongings of the victim such as toothbrush, hairbrush or other items which may be used for comparison purposes in assisting with establishing identity.
The use of human dentition is a common method of human identification. The positions and numbers of restorations in a person's mouth vary and allow forensic dentists to make identifications. More recently, DNA from teeth has been used in much the same way as bone samples. In circumstances where dental records do not exist or cannot be located, DNA analysis can be performed on pulp extracted from the tooth. This is because the hard enamel surface of the exterior structure of teeth serves to protect the internal pulp (Figure 6). For such analysis, individual teeth are typically extracted from a skull (Figure 7), surface sterilized, and pulverized to release the DNA rich pulp.
Saliva itself is not composed of cells but may contain DNA from epithelial cells lining the mouth or from white blood cells resulting from gum disease. DNA profiling has been successfully completed on saliva obtained from sealed envelopes, stamps, cigarette butts, cups, bottles, food, telephone mouthpieces, and bite marks on victims of violent and sexual assaults. The World Trade Centre bombers were identified in part from saliva samples left on stamps.
DNA evidence recovered from a cigarette butt was key evidence used in convicting Michael Feeney whose initial conviction for murder was overturned by the Supreme Court of Canada (SCC). However, after the court ordered a new trial, the police re-investigated. The cigarette butt had not been excluded from evidence. A DNA warrant was obtained and ultimately assisted in Feeney's conviction.
Cases of interpersonal violence, such as assaults, homicides, and domestic violence may include human bite marks.21 In these circumstances, both victim(s) and assailant(s) may sustain bite marks from violence directed at the victim or defensive actions from the victim towards the suspect.
Saliva deposited on victims or suspects in such circumstances can be of significant evidentiary value, assuming it has not been washed away. The collection process is straightforward and involves simply swabbing the bitten area with sterile cotton swabs (one wet followed by one dry). Collection of salivary samples (buccal swabs) requires only gentle swabbing of the inside of the mouth along the cheeks and tongue in order to acquire epithelial cells. (Note: simply dipping the swab in saliva may not yield enough cells for sampling. In these circumstances, contamination may occur, and an emphasis must be placed on the proper drying and storage of the swabs.)
Hair/ Hair Root
Canadian DNA legislation specifically provides for the plucking of hair for DNA analysis. The root sheath-visible to the naked eye as a ball at the tip of a root hair-is a rich source of cellular material and is ideal for analysis. Where hairs are located at a crime scene or on other physical evidence without the root sheath attached, they should not be discounted for analysis purposes. Hairs without the root sheath may be analyzed using mitochondrial DNA analysis as evidenced in the Tran case. However, where possible, hairs with the root sheath attached should be obtained.
Actively growing hairs such as those that may be pulled from a suspect during an assault may have sufficient root sheath for profiling. Generally, ten or more pulled hairs would be required for a suitable sample. Though the legislation does not identify a specific location on the body from which the hair should be obtained, these are generally removed from the scalp. The very nature of DNA does not require the collection of hairs from specific locations on the body for comparison purposes.
Urine. like seminal fluid. does not contain cells unless sloughed from the urinary tract during urination. Because epithelial cells may be sloughed from the urinary tract, urine should not be discounted for evidentiary purposes. DNA suitable for analysis has successfully been extracted from urine soaked bed sheets.
While feces is not commonly recovered from crime scenes, fecal evidence should not be discounted as a potential source of DNA for analysis purposes. Fecal matter contains cellular material from the gastrointestinal tract, however the bacteria present may render it unsuitable for analysis. At the same time, blood can be expelled with feces and consequently may be of potential evidentiary value. Note that dried or smeared feces may be similar in appearance to dried blood. Forensic identification specialists or the regional crime lab should be contacted for advice.