Chapter 2:
DNA as EVIDENCE
The Dawn of Forensic Serology
Ever since the early 1900s when Karl Landsteiner discovered the A,B, and O blood types, it has been the dream of forensic serologists (scientists who study body fluids) to be able to positively identify the individual source of a small blood stain. Advances were made during the 1930s in discovering new blood types such as the Rh factor and again in the 1970s with the invention of electrophoresis, a new method for separating and identifying some of the variable proteins found in blood that are the keys to classic genetic typing. Proteins comprise the majority of the structural and functional substances that make up our bodies. Application of more and more of these typing systems allowed an increasing percentage of the wrongly suspected individuals to be excluded as the source of an evidence stain.
A major problem with the classical systems that test proteins found in blood, is that very few of these proteins are also found in semen and other body fluids. Because the majority of cases that require genetic typing are sexual assaults, the lack of a definitive set of useful genetic markers in semen has long been a great handicap to the scientific analysis of rape evidence.
DNA is found in almost every cell in the body. DNA typing has greatly expanded the sources of evidence that can be tested, while simultaneously reducing the amount of evidence necessary to perform a conclusive test.
Sources of DNA for Testing
Blood
The surface or substrate on which a bloodstain is found can profoundly affect the ability to successfully perform an analysis. Extracting DNA from dried blood found on glass, metal, hard plastics or lightweight cloth is straightforward. Denim, vinyl, carpet, automobile seats and other dense and heavily colored fibers require additional steps. They often contain substances that inhibit the DNA testing process and which must be removed by a variety of purification methods. Concrete and other similar porous surfaces are difficult simply because it is difficult to separate the blood from the concrete. Soil is an almost impossible substrate from which to obtain usable DNA because current extraction methods are not capable of releasing the DNA from soil.
Blood stains may be mixtures of blood from two different people and can produce DNA profiles that are more complex than those from a single individual. DNA profiling may be the only way to determine if a given stain is a mixture. DNA testing can also determine if specimens are from individuals of a different sex. Gender is the only physical characteristic that can be determined by forensic DNA analysis.
Specimens drawn from suspects or victims are called known exemplars and usually consist of liquid blood. The simplest and most reliable samples to work up are liquid whole blood. While the best storage method for blood is frozen, DNA has been successfully isolated after refrigeration for several months. The most common way in which forensic blood samples are handled, following collection, is as a series of dime-sized stains on washed cotton sheeting. This method makes for convenient transport and storage of large numbers of samples.
Occasionally buccal (inside cheek) swabs are supplied as exemplars. All U.S armed forces recruits give both blood and buccal cell samples. This process has the advantage of being non-invasive. It can be used with infants to determine paternity and with other individuals who cannot have blood drawn for medical or religious reasons.
Exemplars may also be obtained from the "Guthrie Cards," widely collected at birth for newborn screening of genetic disease and saved by many states. These have been used in body identification. Similarly a seventeen-year-old band-aid was used to determine the paternity of a young man killed in a car wreck. The question of paternity came up after he was buried, as part of a dispute over insurance benefits.
Semen
Semen stains are the most common evidence to be submitted for DNA analysis and are typically examined and tested by conventional methods prior to DNA analysis. Sperm cell specific staining and microscopy help determine if a stain is the result of vaginal drainage. Vaginal swabs or stains resulting from post-coital drainage will typically contain sperm cells mixed with vaginal cells. Sperm cells as dried stains on furniture or clothing may be identifiable for years.
Semen and condoms are surprisingly common in many environments. Identification of one or a few sperm cells, for example, is not considered highly probative. Sexually active women will often have sperm present in the vagina for 72 or more hours following intercourse. That is why it is useful to also test the boyfriend or consenting sexual partner of rape victims.
Mixtures of cells are usually a confounding factor when performing standard blood typing. In the case of DNA analysis, the process of differential lysis (a controlled disintegration of the cells), is used to separate the sperm cells or male fraction from the non-sperm cells, "the female fraction."
Tissues
Isolation of DNA from tissues taken at autopsy is a simple, straightforward process. More often, soft tissues received in the laboratory are from partially decomposed bodies. DNA survival times are low in soft tissues such as liver and kidney, longer in muscle and brain, and longest yet in dense bone and teeth.
Minute remnants of brain or other tissues that have been scattered by gunshot or other trauma or tissue that might still adhere to the weapon or a bullet can sometimes yield sufficient DNA for testing. Bits of tissue such as these can be used if a body is not recovered and then identified by "reverse paternity," by comparison with surviving family members.
Chemically Treated Tissues
The two common chemical treatments are formaldehyde fixation for medical purposes and embalming. DNA can be successfully tested from both tissues. Bodies that have been buried for several years can even yield results. Most commonly, exhumations are required in paternity determinations, but they may also provide a known exemplar to compare with evidence in a forensic case.
Hair Roots
One to five hair roots can contain sufficient tissue for RFLP analysis. Shed, or telogen, hairs contain only trace amounts of DNA, and are generally not suitable for testing by current methods in routine forensic use. A highly specialized method known as mitochondrial sequencing is beginning to be used more and more. It was this method that was recently used to identify one of Napoleon's hairs and members of the Romanoff family.
Saliva
DNA can be typed from saliva deposited on envelope flaps or stamps, as was done in the New York World Trade Center bombings. It can also be taken from cigarette butts found at crime scenes, and even from cups, bottles, telephone mouthpieces, bite marks and penile swabs.
Urine
Success with DNA analysis of urine is less common, as healthy individuals do not shed nucleated cells into the urine. Conventional testing is more likely to yield results, and is used in disputes over samples that have been tested for drugs.
Products of Conception
The non-living products of conception most often analyzed are abortuses. Usually these cases stem from allegations of criminal paternity where a teenaged girl has been impregnated by a relative or other adult. Other cases might involve a prison guard accused of having sexual relations with an inmate or a rape resulting in pregnancy.
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