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MCLE Self Test
DNA's Practical Applications
By Laura Gahn and Jennifer McCue

Edited by Barbara Kate Repa
Category: Evidence

        Although the O. J. Simpson trial first shone the spotlight on the use of DNA evidence in the courtroom, many recent high-profile criminal cases, including the California kidnapping and murder of Samantha Runnion, have relied heavily on such evidence to identify a suspect and to convict or exonerate the accused. With the increasing popularity of DNA analysis in the courtroom, it's essential that lawyers-particularly those in criminal practice-understand the basics of DNA identification and its practical applications.
        Overview of DNA
        There are billions of base pairs or pieces of DNA located throughout each human cell. DNA determines our genetic traits and characteristics-including gender, facial features, and hair and eye color.
        More than 99 percent of one individual's DNA is the same as every other human's DNA. Testing, as part of a criminal investigation or prosecution aimed at identifying an individual, focuses on the unique 1 percent or so of the DNA that varies. Identical twins are the only individuals who contain the same set of DNA; there is no way to genetically distinguish between the two.
        Since DNA is unique to almost everyone, identification testing can be useful in matching evidence from a crime scene or victim with a suspect, determining the identity of an individual or of human remains, or determining biological relationships in a paternity case.
        Forensic DNA Test
        Until recently, forensic DNA identification testing has been used in cases in which investigators or law enforcement agencies had already identified a suspect. In 1990 the Federal Bureau of Investigation started a pilot program called the Combined DNA Index System, or CODIS, which operated in 14 states. CODIS was developed as a way for federal, state, and local governments to exchange and compare DNA profiles electronically. In 1994 the DNA Identification Act was passed, which granted the FBI permission to begin a national DNA database for law enforcement purposes. 42 USC §14131. The database became fully operational in October 1998.
        The CODIS database contains DNA profiles from convicted offenders, as well as DNA evidence from previously analyzed violent crimes. When DNA evidence is obtained from an unsolved crime, a DNA profile can be determined and submitted to the database. Matches of current crimes to either a convicted offender or to previous crimes are called hits. Hits can provide investigators with further information about connected crimes or even a suspect, which can result in an arrest and possible prosecution of a case that may have little or no other evidence. Because of the uniformity of DNA testing nationwide, database hits may occur in cities or even states other than where the crime occurred. This system is invaluable in determining patterns of violent crimes or linking crimes to the perpetrators.
        By convention, forensics laboratories in the United States now test 13 Short Tandem Repeat (STR) systems, also referred to as CODIS loci. Loci are known to vary in the population enough to establish identity to a high degree of certainty. By using the same 13 CODIS systems and standardizing the way results are reported, it is easy to compare results from different laboratories.
        California Legislation
        Each state has different laws controlling which samples and offenders are placed in the national database. In September 2000 California passed several laws that strengthened the use of DNA evidence in the courtroom.
        One of the new measures granted California law enforcement permission to use DNA samples for solving serial crimes; it allows DNA samples of charged criminal suspects to be compared against all unsolved crimes. Pen C §299.5. Another law granted the California Department of Justice permission to create a DNA database for identifying missing persons. Pen C §14250. That database is kept separate from the national convicted offender database and contains DNA profiles from unidentified bodies. These DNA profiles can be cross-referenced with profiles from individuals abducted by strangers or from missing persons involved in high-risk situations.
        One of the most important laws for lawyers to be familiar with provides imprisoned felons with the opportunity to request DNA testing in a motion for a new trial. Pen C §1405. That law requires the felon to specify how the testing would raise a reasonable probability that the verdict or sentence would have been more favorable if the DNA test results had been available at the time of conviction. The judge who conducted the defendant's trial is required to hear the motion, but he or she may deny the motion without a hearing.
        Biological Evidence from a Perpetrator
        The most common kind of case that includes evidence suitable for DNA analysis is sexual assault. Vaginal, anal, and oral swabs often contain the perpetrator's semen mixed with skin cells from the victim. Special procedures can be used to extract DNA from the perpetrator's semen and compare it to a suspect's DNA sample. If there is no suspect in the case, the sample can be compared to the CODIS database to search for a match. This DNA evidence can also be extracted from a stain on bedding or clothing found at the crime scene.
        Other kinds of a perpetrator's biological material may be compared to the CODIS database or used to link a suspect to the crime. Without being aware of it, a perpetrator may leave behind skin cells, hair, or blood on the victim. Or a perpetrator may smoke a cigarette or drink from a glass and leave behind valuable saliva DNA evidence-all of which can be used to convict or exonerate a suspect.
        Forensic DNA testing can also be used to place someone at a particular location within a crime scene-a useful tool for crime scene reconstruction. A laboratory can examine a piece of evidence that contains DNA and compare it to the victim's and the assailant's DNA to determine identity. That is especially useful in driving under the influence cases to determine which party may or may not have been driving.
        Biological Evidence from a Victim
        Investigators may also search a suspect's body or belongings for traces of the victim's DNA. Hair, blood, and other biological materials from victims may be found on the suspect, on clothing, or in a suspect's vehicle or residence. Such material may be the only physical evidence linking a suspect to a particular victim or crime scene. That type of evidence is common when there is a violent fight, assault, or homicide that involves blood, or scratching or scarring skin.
        Identity Determination
        Although DNA evidence from a perpetrator or victim is useful in determining if a suspect can be connected with a crime, DNA analysis is also commonly used to determine the identity of human remains. The success of DNA typing from a corpse varies widely, depending on the condition of the body.
        Two methods are commonly used: comparing the DNA sample from the corpse to a known reference sample, or comparing it against a DNA sample from a possible biological relative.
        A reference sample is a sample of known origin that can be compared to an unknown sample to see if there is a match. A reference sample can be a cheek swab sample, an unusual DNA sample such as one taken from a toothbrush or hairbrush, a previous biological sample taken for medical purposes, or a DNA sample obtained from a biological relative.
        When available, known DNA samples from a corpse can be compared against a DNA reference sample to determine identity. If a sample from a corpse matches a known reference sample, the investigator has positively identified the body. If a sample from a victim does not exist, blood relatives such as a biological parent or child of a missing person can be used instead. By comparing the purported relative's DNA sample against the DNA sample collected from the corpse, the laboratory can determine if the two individuals are biologically related.
        The most complex and challenging real-world example to date of an attempt to identify remains by DNA testing is the World Trade Center victim identification project. After the terrorist attack of September 11, 2001, that resulted in the deaths of thousands of individuals, the state of New York created a database containing samples from individuals who were searching for biological relatives thought to be victims of the attack. Investigators collected DNA samples from the wreckage and compared them to the samples in the database. If a match occurred, the investigators were able to genetically determine the identity of the victim and bring closure to the family.
        DNA paternity testing also focuses on testing that unique 1 percent of DNA that varies in the population to positively identify individuals. Each laboratory determines which and how many DNA markers it will test. Unlike the CODIS markers used in forensic DNA analysis, paternity-testing laboratories do not use a standard set of markers, so results are not always comparable from lab to lab.
        The discriminating power of a paternity test can also vary depending on the number of genetic markers used and on the specific DNA profiles of the individuals tested. Generally, the more markers used, the greater the probability of paternity in the test results. Although the minimum probability of paternity requirements for court-admissible paternity tests vary from 95 percent to 99.9 percent depending on the state, California requires a probability of paternity of 99 percent or higher.
        In cases in which an individual is a victim of a sexual assault and gives birth to a child as a result, paternity testing can be especially useful in determining whether the child is the biological child of a suspect. The test results can then be used to convict or clear him.
        Legal test results. In legal paternity tests, it is important that an impartial third party collect the tested parties' samples. In legally binding cases, it is especially important for laboratories to verify that the proper procedures have been followed throughout the testing process-including strict chain of custody guidelines during the collection procedure, while the samples are being transported to the laboratory, and following the testing process. If the proper procedures are not followed, the laboratory may not be able to use the samples, causing possible delay in processing the test results.
        Sample type. A range of sample types can be used for paternity testing. The most commonly used DNA samples are collected from the skin cells on the inside of an individual's cheek. These cells are easily removed with a simple buccal swab, which resembles a long-handled Q-Tip. DNA samples can also be extracted from an individual's blood, hair, or from other biological materials if needed.
        Test process. Half of a child's DNA comes from the biological mother and half from the biological father. During the standard testing process, the child's DNA is compared to the DNA from both the mother and the alleged father to determine the probability that the alleged father is the biological father. However, a paternity test can still be performed without a sample from the mother. In those situations samples are collected from the child and alleged father and compared to determine the probability of paternity. Although there are advantages to including the mother in the testing process, paternity testing can be performed when the mother is not available for testing.
        Y Chromosome and Mitochondrial DNA Tests
        Two other types of DNA testing are becoming commonplace. The first is Y chromosome DNA analysis, which is used to determine only paternal lineages or to isolate and identify a male DNA profile found in a male/female mixture-common in sexual assault cases in which evidence may contain a mixture of both male and female biological material. Mitochondrial DNA testing is commonly used to determine maternal lineages only. Mitochondrial DNA testing has the added advantage of giving results on some samples that are not amenable to standard STR DNA analysis. That type of testing is often used when samples are compromised due to age, or with samples that are the result of a mass disaster such as the World Trade Center attack or an airplane crash.
        Selecting a DNA Testing Laboratory
        Lawyers have a number of choices when selecting a public or private laboratory to send cases for DNA analysis. Accreditation and turnaround time are two important points to keep in mind when selecting a lab.
        Accreditation. Forensic laboratories gain accreditation from either the National Forensic Science Technology Center or the American Society of Crime Laboratory Directors' Laboratory Accreditation Board. Accreditation establishes that the forensic DNA testing laboratory complies with national quality-assurance standards set forth by the director of the FBI's DNA Board. During the accreditation process, auditors review the laboratory's policies, practices, and procedures to determine whether they meet the board's strict standards. That includes verifying standard operating procedures, validation studies, proficiency tests, management practices, equipment calibration, and reviewing training and education of the laboratory staff. That review also includes an examination of the paperwork and records and involves a thorough inspection of the facility and personnel.
        The American Association of Blood Banks (AABB) also evaluates and accredits paternity-testing laboratories for their compliance with a strict set of DNA testing standards. To gain AABB accreditation, a paternity lab must follow a list of rules and policies that outline proper sample collection, chain of custody procedures, and test-result reporting. Strict quality-assurance standards must also be followed.
        Turnaround time. Standard turnaround time varies greatly, depending on the laboratory and the specific testing requested. Most paternity-testing laboratories report test results within 3 to 14 days, with options for rush testing when necessary. Turnaround times for forensic DNA cases can also vary greatly-testing can take anywhere from as little as two weeks to several months, depending on the laboratory and the work required.
        Limitations of DNA Testing
        DNA testing is technically demanding and requires both sophisticated equipment and staff who have extensive training to ensure proper testing procedures and accurate results.
        Not all biological materials are suitable for DNA testing. Evidence that is exposed to elements such as heat or moisture will degrade, producing only partial results, or no results at all. Biological stains that have already dried on cloth, concrete, or other materials are relatively stable for testing. When dealing with limited remains, long bones or teeth-as opposed to other bone samples-have the highest likelihood of success and can undergo standard STR analysis. Hair samples without an intact root may only be used in mitochondrial DNA testing.
        Although DNA identification can show with a high degree of certainty whether biological evidence is from a specific individual, it says nothing about how or when that evidence came to be. As a result, DNA testing must be used in conjunction with other investigative tools.
        Laura Gahn is DNA laboratory director at Identigene, a national DNA identification company located in Houston that specializes in paternity and forensic DNA testing. Jennifer McCue is a forensic DNA analyst at Identigene who frequently acts as an expert witness in DNA testing. Both can be reached at

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