The DNA Fingerprint and Criminal Evidence

Advanced Preparation

Teachers will need to familiarize themselves with the materials and resources listed below. It would be a good idea for the teacher to observe and/or participate in the Gel Electrophoresis lab to get an idea of the procedures that biologists and chemists use to test DNA samples. Also, the teacher should examine or participate in the math probability lesson.


In 1988 the DNA fingerprint was first admitted as evidence in court in the case of Florida v. Tommy Lee Andrews. In January 1989, the Federal Bureau of Investigation (FBI) began to accept casework from state forensic labs. Since then DNA finger printing has been used in hundreds of cases in the United States and has been formally allowed in at least one jurisdiction in about two-thirds of the states.

As a new technology, DNA fingerprinting had to be found in each of the courts to satisfy well-established standards for the admissibility of novel scientific evidence. The most common standard is known as the Frye rule. The rule is named in recognition of the 1923 federal court case of James Frye, a black man accused of having murdered a white man. Frye's lawyer urged the court to admit in evidence the results of a systolic blood pressure test--an early form of lie detector--on the grounds of the general rule allowing experts to testify on matters of specialized experience or knowledge. Because the polygraph was a new technology, however, the court imposed a more stringent evidence rule. The court, believing that the polygraph did not yet command general acceptance, declined to admit the lie-detector results. Some courts apply a different standard known as the relevance test which is based on the Federal Rules of Evidence, but the question of admissibility of novel evidence is the same.

Because DNA analysis was widely accepted in medical applications, courts quickly decided that DNA fingerprinting satisfied the Frye rule. Underlying most of the early court decisions where DNA evidence was allowed were the following assumptions: (1.) DNA was in every cell in the body; (2.) and therefore evidence samples would be identical to samples from the guilty party; (3.) and that falsely positive identifications would be essentially impossible. Courts have accepted DNA fingerprinting because in theory the procedure is faultless. If enough sites of genetic variation are examined, it is certainly possible to determine whether two samples come from the same source.

In practice, however, DNA fingerprinting presents problems. (1.) In DNA medical diagnosis, testing can be conducted under optimal conditions. DNA samples are fresh and clean and usually from a single individual; if mistakes are made in the analysis, new samples can be acquired and tests redone. In DNA forensics the crime lab is constrained by whatever samples happen to be found at the scene of a crime. Samples may have been degraded and may be mixtures of samples from different individuals, as happens in a multiple murder. Also, the forensic scientist often has only a small amount of DNA, only enough to do one test, and as a result the test cannot be repeated because the sample will have been used up. (2.) DNA medical diagnosis usually asks: which of two alternative RFLP alleles has a parent passed on to his child? Because there are only two possible alternatives, there are natural consistency checks to guard against error. DNA forensic scientists are presented with the situation were they are given two samples related to a crime scene, about which they know nothing in advance, and are asked whether or not they are identical. They first need to determine if the band patterns match, a decision which requires them to make fine judgments about whether small differences between patterns are meaningful. If the scientists decide that the patterns match at a few sites of variation, then they must assess the mathematical probability that the match might have occurred by chance. For this purpose, the distribution of band patterns in the general population must be known by the forensic scientist.

Although DNA evidence in most courts is generally admissible in principle, the analysis of the evidence presented by forensics labs is the area that is brought under careful inspection. In many cases, judges have ruled that DNA evidence was not admissible because the analysis did not follow generally accepted principles of forensic analysis. Also, in dozens of cases prosecutors have chosen to withdraw DNA evidence when defense attorneys have hired their own forensic experts who raised questions about the validity of the evidence.

In this lesson students are asked to become familiar with how forensics labs test DNA samples, how forensic scientists analyze DNA fingerprints, the process by which DNA fingerprints are admitted as evidence, the role that DNA evidence plays in the courts verdict, and the questions that DNA evidence raises in society.

Student Objectives

Class Time Needed

10 days



  1. (1 Day) Determine what the students know about DNA fingerprinting. Ask the students if they have been involved in electrophoresis labs or if the issue has been raised in their math, English, science, or other social studies classes. Show the class an electrophoresis gel to make the point that what is being studied in their biology/chemistry classes will be applied in social studies. Stimulate their interest with the following examples of DNA fingerprinting:
    1. Investigators linked DNA in the saliva used to lick envelope to one of the World Trade Center bombers. The test is possible because bucal cells from inside the cheek, rich in DNA, are deposited with saliva and can be recovered from stamps, glass rims, or even from a killer's deadly kiss.
    2. Paternity suits are commonly settled by DNA testing. Former mayor of Detroit Coleman Young agreed to child support payments in 1992 after testing resolved a dispute case.
    3. Researchers performed DNA tests on lung tissue from a 1,000 year old mummified Chiribaya Indian, and found an exact match with the DNA of the tuberculosis bacterium, thus exonerating Christopher Columbus on the charge that his ships had introduced the White Plague to the New World.
    4. At the Armed Forces Institute of Pathology, a massive collection of DNA samples from every member of the armed services began recently. By late summer of 1994, more than 200,000 individual samples were logged into a computer database and stored in freezers at the nearby DNA Specimen Repository. By the end of 1995, the repository could have more than two million samples. Cards with blood blots from each member of the armed services are sealed in individual envelopes before freezing. They are subjected to polymerase chain reaction (PCR) and other tests only if the need arises. In the system's first two applications, cards were pulled and tested in 1994 to identify a soldier burned to death in a car accident and a pilot killed in a plane crash. A one-eighth inch hole punch taken from a blood blot the size of a fifty-cent piece was enough for the required tests.
  2. (1 Day) Tell them that this class will be involved in an investigation of how DNA fingerprinting is used as criminal evidence in court
    1. A short lecture drawn from the sources listed below would be an appropriate method to introduce/reinforce the following:
      1. What is DNA?
      2. Each individual's DNA is unique: the "fingerprint"
      3. DNA tests have been devised to identify individuals
      4. How DNA tests are analyzed
      5. DNA fingerprint analysis has been introduced in court as criminal evidence
      6. That DNA evidence is controversial
      7. That the class will analyze why DNA evidence is controversial
      8. That the class will determine under what conditions DNA evidence should be used in court
    2. Hand out and explain the student assignment guide
  3. (1-2 Days) Show the first segment from the Nova: Murder, Rape, and DNA.
    1. While students are viewing the video, have them follow the questions provided on Overhead #1.
    2. Stop the video periodically and discuss the questions with the class. Have them summarize their answers in their notes.
    3. At the end of the video have students list their conclusions in their notes to question #11: What conclusions can you reach about the admissibility of DNA evidence in court?
      1. Hold a class discussion about their conclusions.
      2. List the conclusions on the overhead and have student add them to their notes.
  4. (1-2 Days) Begin the period by reviewing the conclusions reached about admissibility of DNA evidence in court . Tell the class they will be watching a video that links DNA evidence with the O.J. Simpson trial.
    1. Show 60 Minutes/DNA.
    2. While students are viewing the video have them follow the questions provided on Overhead #2.
    3. Stop the video periodically and discuss the questions with the class. Have them summarize their answers in their notes.
    4. At the end of the video have students list their conclusions in their notes to question #10: What new conclusions about the admissibility of DNA evidence in court have you reached
      1. Hold a class discussion about their conclusions.
      2. List the conclusions on the overhead and have student add them to their notes.
  5. (5 Days) Students will participate in a mock hearing. The situation is the following: The U.S. court system has been placing increasing pressure on forensic laboratories and the legal profession to address the controversial issue of DNA fingerprinting analysis and its use as evidence in court. The National Association of Trial Lawyers (NATL) has directed its policy making committee to hold hearings among its members throughout the country for the purpose of coming up with recommendations that will state the conditions under which DNA evidence will be allowed in court. Three representatives from the NATL have arrived in your city and will hold a hearing in three days to come up with a series of recommendations. As members of the association you will participate in the hearing by arguing recommendations that you want to see included as part of their report.
    1. Choose three students to act as the panel. The group you select must have good organizational and questioning skills. They should work separately from the rest of the class and follow the procedures outlined in Student Handout #2: DNA Evidence Hearing Guidelines.
    2. The rest of the class will act as the NATL membership from your community. There job is to research and come up with recommendations for the panel. To facilitate discussion and the research process, students should be broken into groups of three students. The procedures for the membership are outlined on Teacher Overhead #3: The Situation.
    3. After each group has researched and brainstormed recommendations that they can support with evidence, each group should produce at least three recommendations. As students complete their Student Handout #1: Recommendation Forms, critique them for clarity, logic and the appropriateness of the sources of information.
    4. After students have completed their research and recommendations plan out with the class the hearing procedures. Go over the hearing agenda on Student Handout #3 with the hearing committee panel. With the rest of the class tell them the following:
      1. panel members will call for recommendations and students will raise their hands for recognition. Once they are recognized they will go to the podium and give their recommendation and evidence to support their recommendation. The presenter may use handouts, graphics; make use of the overhead projector, chalkboard, or butcher paper.
      2. after each recommendation is presented the panel and the other students in class will raise questions about the recommendation. Is the recommendation clearly stated? Is there adequate evidence to support the recommendation? Is there evidence that contradicts the recommendation and evidence? Should additional evidence be added to the recommendation?
      3. all recommendations will be written down by a panel member for all members of the class to see.
      4. the hearing will continue until all recommendations are heard. ]Note: You should anticipate that there will be at least three recommendations. If students come to the podium and present a recommendation that has already been presented, see if the panel responds. If the panel does not respond, ask the student presenter how their recommendation compares with the recommendations already submitted.]
      5. all students are expected to participate, either as panel members, presenters, or students participating in asking questions or discussing the recommendations.
    5. You will need two to three days for research and writing recommendations and one or two days for the hearing and announcement of the hearing panel's decision
    6. In preparation for the hearing, set up the room as diagrammed in figure 1.

Resources and References

Daniel J. Kevles and Leroy Hood editors. The Code of Codes: Scientific and Social Issues in the Human Genome Project. (Cambridge, Massachusetts: Harvard University Press, 1993), pp. 193-196.

"DNA's Greatest Hits" in Popular Science. November 1994, pp. 64.

"Text of Superior Court Judge Lance Ito's Ruling on DNA testing of blood evidence, August 26, 1994," Court TV, America Online, August 26, 1994.

"What is the law in California about the use of DNA evidence? Court TV," America Online, February 9, 1995.

"A look at the prosecution's DNA evidence in the Simpson Case," Court TV, America Online, May 5, 1995

"High Profile: The Simpson case raises the issue of DNA reliability," Scientific American. October 1994, Vol. 271, No. 4, p. 33.

Seand Henahan, "An Interview with DNA Forensics Authority Dr. Bruce Weir," Access to Excellence, American Online, January 11, 1995.

"Answers to some of the frequently asked questions about DNA evidence," Court TV, America Online, August 3, 1994.

Saltus, Richard. "DNA FINGERPRINTING: It's a case of probabilities," The Boston Globe. August 22, 1994, p. 25.