Search NOVA Teachers

Back to Teachers Home

Lost Tribes of Israel

Classroom Activity

To understand the issues involved with using DNA evidence in a courtroom trial.

Materials for each team
  • copy of "Did the Dog Do It?" student handout (HTML)
  • Additional reference sources
  1. Organize students into two juries of approximately 12 students each. Distribute copies of the "Did the Dog Do It?" student handout to each student.

  2. Tell students they will be trying a case involving a man bitten by a dog. A pit bull named Buddy is accused of biting a 38-year-old man named Taylor. The pit bull's owner, Sam, says his dog didn't do it. Taylor claims that DNA taken from saliva on a towel he used to clean the bite matches Buddy's DNA.

  3. Have students read the evidence that has come out during the trial and discuss what they can infer from this evidence.

  4. Tell students to brainstorm other questions they would like to have answered about how DNA fingerprinting works. Instruct students to use additional resources to research their answers.

  5. Have students consider what other questions are important in deciding this case, such as how the evidence was collected and processed.

  6. Based on what students learn from their research and the other issues they have considered, have them vote and decide whether they think the accused canine is innocent or guilty.

  7. Have each jury report its verdict and explain how it reached its decisions, including what resources it used to come to its conclusions. If either jury is hung, discuss why and whether additional information would have helped them reach a verdict.

  8. Discuss with students the differences in how the DNA is used in the program and how it is used in this activity. (The DNA evidence used in the program looks at a set of unique changes on the Y chromosome over time; however, those changes may not be present in Jewish males who are descendants of the Cohanim, or may be present in Jewish males who are not descendants of the Cohanim. The DNA markers in this activity are used to determine the probability of a match to a specific individual dog, as well to exclude dogs who do not have the match.)

Activity Answer

DNA evidence cannot conclusively prove that a person, or in this case, a dog, committed a crime. What it can do is show the probability of someone having the same DNA match. The probability given for a DNA match states the probability of finding a particular profile by chance in a population. For example, if the probability of 1 in 10,000 were given for a match to the dog's DNA, then in a city of 5,000,000 dogs, there would be 500 dogs that could match this profile purely by chance. Jurors would then need to decide whether the dog is innocent given this probability.

DNA evidence can also rule out people from being considered as suspects when no match exists.

Every organism's DNA is composed of strings of four different nucleotides: G(uanine), C(ytosine), A(denine), and T(hymine). These strings of nucleotides are connected to one another by nucleotide pairing (G-C and A-T) to form the two-stranded DNA molecule that makes up the chromosome. For the most part, the order—or sequence—of these base pairs is very similar from one individual to another. However, there are regions of DNA that are highly variable in length and/or sequence and therefore are different from individual to individual (except in identical twins whose DNA is identical). These variable regions of DNA are typically used in DNA fingerprinting.

A DNA fingerprint is made by taking a sample of DNA—which can be taken from nuclear or mitochondrial DNA found in almost every living cell—making copies of the extracted DNA, and isolating certain known base pair sequences. Since the fragment lengths starting with these known sequences differ in every person, they can be used to help determine identity. A DNA fingerprint looks at only a small number of base pair sequences contained in a person's total DNA. Nevertheless, the differences between the DNA in different people is such that even these small number of sequences can eliminate a large majority of other people as a suspect.

Other issues students might consider:
How was the DNA collected and processed? Could the evidence have been contaminated with DNA from another source? How much DNA was available for testing? How many different DNA segments were analyzed? Are any other neighborhood pit bulls from the same litter as Buddy? Were the lab procedures conducted accurately? What were the credentials of the expert who presented the DNA evidence? Was the expert paid, and if so, how much?

Links and Books


Journey to the Vanished City by Tudor Parfitt, London: Hodder and Stoughton, 1992.
Delves deeper into the journey described in the program and includes features such as a map of the route.


"Order in the Lab! As the judge sets a date for the Simpson trial, lawyers wrangle over the DNA tests that could seal O.J.'s fate." by Leon Jaroff, Time, 8 August 1994, 46.
Reviews different types of DNA processing techniques.

"The Priest's Chromosomes." by John Travis, Science News, 3 October 1998, 218-219.
Details the DNA analysis supporting the passing of a genetic marker on the Y chromosome of Jewish priests from father to son.

Web Sites

NOVA Online—Lost Tribes of Israel
Delves deeper into the program's content and themes, with features such as articles, timelines, interviews, resource links, and more. Includes an online activity on how to create a DNA fingerprint. Launch date: Friday, February 18.

Basics of DNA Fingerprinting
Explores what DNA fingerprinting is, how it is done, its applications, and some of the problems with using it.

Blackett Family DNA Activity
Details the concepts and techniques behind DNA profiling, interpreting DNA autoradiograms, and evaluating DNA profiles to determine familial relationships.


The "Did the Dog Do It?" activity aligns with the following National Science Education Standards:

Grades 5-8

Science in Personal and Social Perspectives

Science Standard F:
Science in Personal and Social Perspectives

Science and technology in society

  • Science influences society through its knowledge and world view. Scientific knowledge and the procedures used by scientists influence the way many individuals in society think about themselves, others, and the environment. The effect of science on society is neither entirely beneficial nor entirely detrimental.

Grades 9-12

Science in Personal and Social Perspectives

Science Standard F:
Science in Personal and Social Perspectives

Science and technology in local, national, and global challenges

  • Understanding basic concepts and principles of science and technology should precede active debate about the economics, policies, politics, and ethics of various science- and technology-related challenges. However, understanding science alone will not resolve local, national, or global challenges.

Teacher's Guide
Lost Tribes of Israel

Video is not required for this activity