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DNA on Trial
Grades 9-12

This lesson is used to introduce issues that arise when DNA fingerprinting is used as evidence.
Green Means #201: Crimes Against Nature
NOVA: Murder, Rape and DNA
College Algebra #26: Probability

Students will understand:
· That DNA fingerprinting can be used to eliminate a suspect from a crime.
· The controversy around using DNA as legal grounds in the American court system.
· How probability of an event is determined.


Explain to students that each individual organism has it's own specific combination of DNA. In fact, the information is so specific that these "fingerprints" are used in courts of law to prove or disprove the guilt of certain suspects.Direct students to brainstorm how DNA fingerprinting might be used in a variety of fields: example - protection of wildlife, determining hereditary disease, guilt or innocence in a court of law. Use the GREEN MEANS video to introduce applications of DNA fingerprinting.

Ask students if it is possible to prove the innocence of a suspect based solely on the DNA evidence.

PLAY the 4 minute video "Crimes Against Nature" from the beginning. Instruct students to focus on how DNA is used by the Federal Fish and Game Department. They should record this information in their journals. (DNA fingerprints are used to prove that a certain animal was poached from Clint Eastwood's land.)

SHOW the first 20 minutes of NOVA: Murder, Rape and DNA which involves a father who is accused of the rape of his daughter. DNA samples from him and evidence (semen) do not match.Therefore, he is eliminated as the suspect.

SHOW the video College Algebra #26: Probability. Segmented viewing: Tape should be cued to the segment about finding somebody with the same birth date. (Eight minutes into tape.) This is a good explanation of probability. PAUSE the tape. Ask students to predict the results in their class. Then survey the class for shared birth dates.

Review students' responses. Student challenge: Can a single fingerprint prove that there is an exact match between two samples? (The answer is NO, because each sample is simply a minuscule part of an organism's genome. A human's is 60 feet long when extended. Scientists look at a portion less than .25 inch at the most.)
Discuss any other instances or cases in which DNA has been used as evidence.

Compare the results in both videos. Discuss how it takes ONLY ONE sample location that does not match to eliminate a suspect.
Ask students if they could guess how many matching samples it might take to prove guilt. Lead a discussion of the probability of an event.

Review the fundamentals of probability, giving examples of coin tossing. For example, what is the probability that two coins tossed simultaneously will be TT? (1/2 times 1/2 = 1/4)
Ask again, how one would determine if matching DNA samples would prove somebody's guilt? How many samples would have to match?
Explain that this is the exact question that many legal courts are wrestling with right now. The type of testing (PCR or RFLP makes a big difference.) You may want to have students read "How DNA Fingerprinting Works," an excerpt from an article from the November, 1994 issue of Popular Science, to help explain the difference.
Unfortunately, there is disagreement between probability of matches. For example, the probability of two matches between two blacks is greater than that between a Black and an Asian.

Given the probability of an event occurring is a product of each individual event, what would the probability be that there would be a match between two samples if the probability of each of five matches for RFLP testing was:
1. 1 in 2 (1/2 to the 5th power, or 1/32)
2. 1 in 5 (1/5 to the 5th power, or 1/3125)
3. 1 in 10 (1/10 to the 5th power, or 1/100,000)
4. 1/15 (1/15 to the 5th power, or 1/759,375)
5. 1/25 (1/25 to the 5th power, or 1/9,765,625)
Cited range of the probabilities for RFLP analysis's is one in tens of thousands to one in hundreds of thousands, or even millions for five locations to match. Ask students to calculate the probability of each individual match for the following:
6. five matches = 1 in 1,000 (about 1/4 each event)
7. five matches = 1 in 33,000 (about 1/8 each event)
8. five matches = 1 in 250,000 (about 1/12 each event)
9. five matches = 1 in 760,000 (about 1/15 each event)
10. five matches = 1 in 1,800,000 (about 1/18 each event)
11. five matches = 1 in 3,200,000 (about 1/20 each event)

FAST FORWARD tape where explanations are given as to how to determine the probability of an event. (Number of positive outcomes/number of total outcomes.) PAUSE tape and relate this discussion to DNA use as evidence.
Next, SHOW the segment where the probability of an event consisting of numerous events = probability of each event times each other.


1. Discuss which probabilities would be acceptable in a court of law to alone prove guilt. (Remember, other evidence frequently enters into the trial. DNA alone may not be necessary to prove guilt.)

2. Discuss how other circumstances, such as the source of the samples (general population, Blacks, Whites, Asians, etc.) will change these probabilities.

3. Discuss what needs to be done in order to determine a number where probability can be used to prove guilt (standardized tests of each ethnic group, or standardizing the gene pool sample by location, for example.)

4. Discuss how much of the human genome is actually being sampled (very small portion) and why more samples cannot be taken. (It is a difficult procedure finding these sites within the whole genome.)

5. Discuss the highly technical and complicated nature of this science, and how universal agreement as to interpretation of results is not determined.

· Connect with an on-line service to investigate current DNA technology. Use keywords such as DNA and biotechnology. Is it possible to test more sample locations? Genentech has a service on America OnLine.
· Using the on-line service, investigate how our legal system uses DNA fingerprinting in courts. AOL keywords would include Court TV, CNN, forensic science, reference desks and DNA. Under "Court" there are current rulings on DNA standards that various courts are presently following.
· Students can experience an interview with a real-life Forensic Scientist by visiting NewsMaker on the World Wide Web. Dr. Bruce Weir, noted DNA expert from the OJ Simpson trial, is interviewed on some of the most asked questions stemming from "The Trial of the Century". Questions which students can answer from the interview include:
1. Where does DNA evidence come from?
2. What is the difference between PCR and RFLP methods of DNA profiling?
3. What statistical techniques are used to confirm the reliability of DNA evidence?
4. What role does race and ethnicity play in the accuracy of DNA evidence?
5. What is the effect of human error in the preparation of DNA evidence?
6. Why is the validity of DNA forensics and the statistical accuracy of DNA profiling in question?

Master Teachers: Stan Hitomi and Randall Lam

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