UNIT: Forensics Unit
GRADE: 11th Grade
CLASSES: Biology and Math III
DURATION: 75 minutes
Lesson Plan DNA Probability
OBJECTIVE OF THE LESSON: Students will be able to calculate the combined genotype
frequency of a fictitious DNA profile.
STATE BIOLOGY and PROBABILITY STANDARDS APPLIED:
Biology:
• Mutation and sexual reproduction lead to genetic variation in a population.
• Genes are a set of instructions encoded in the DNA sequence of each organism that
specify the sequence of amino acids in proteins characteristic of that organism.
• Students know how to solve the Hardy-Weinberg equation to predict the frequency of
genotypes in a population, given the frequency of phenotypes.
Probability:
• Students know the definition of the notion of independent events and can use the rules for
addition, multiplication, and complementation to solve for probabilities of particular
events in finite sample spaces.
MATERIALS:
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Dice for each student pair
A loci card for each student pair
A calculator for each student pair
A Fictitious DNA Profile worksheet for each student pair
One copy of the Journal of Forensic Science (Butler et al., 2003) article for each student
One copy of the Table Talk questions for each small group (4 students per group)
PREPARATION FOR THE LESSON: Seat students in small groups of 4 or 6. Have each
student sitting next to a partner.
METHOD OF INSTRUCTION:
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Introduce the lesson: Explain the results of the Forensics PCR and Gel Electrophoresis
lab. Explain that students will be learning how to use laboratory DNA results to calculate
the probability that the DNA sample matches no other person in the world except the
suspect (or the victim depending). Forensic scientists connect the DNA to the person and
putting that person at the scene (placing a suspect at the scene of the crime or placing a
victim at the same location as a suspect). What DNA did we test and why?
Developed by Jesse Wade and Amy Callahan
High Tech High, San Diego, CA
July 8, 2008
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Activate prior knowledge: Ask students to look at the results of the lab. Ask students
why there were two bands for each DNA sample in the gel. Ask students what the bands
represent. Think Pair Share.
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Guide group discussion: Each suspect had two alleles because each person has a pair (2)
of each chromosome. All organisms get one allele from mom and one allele from dad.
Show slide of 23 pairs of human chromosomes (Figure 1).
Figure 1. Picture of the 23 pairs of chromosomes in humans (Access Excellence, 2008).
We tested just one loci on one chromosome. Which loci did we test? We tested TH01.
Show the slide of 13 CODIS Core STR Loci (Figure 2). Looking on the loci map, which
chromosome did we test? We tested chromosome 11. How many other loci are there on this
map? There are 13, but there are other loci that the U.S. does not test. Why do you think
they test 13 loci? We will see today why they test only 13 loci.
Figure 2. 13 CODIS STR Loci (Butler & Reeder, June 23, 2008)
Developed by Jesse Wade and Amy Callahan
High Tech High, San Diego, CA
July 8, 2008
How many possible allele outcomes were there on our TH01 loci? There were 8 possible
alleles. If there was an equal probability of getting each different allele, what is the
probability of getting allele 10? You have a 1/8 chance of getting allele 10. What is the
probability of getting allele 5? You have a 1/8 chance of getting allele 5. What is the
probability of getting allele 7? You have a 1/8 chance of getting allele 7. What about having
a 10 and 5? (1/8 x 1/8) x 2 because you could either have 10, 5 or 5, 10.
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Pass out materials: Pass out dice, a loci card, and A Fictitious DNA Profile worksheet for
each student pair.
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Explain the Fictitious DNA Profile activity: We will be modeling how the FBI tests 13
loci to determine that the DNA sample is a match with the person. Today we will be
oversimplifying this process. Each of you will determine the alleles for one locus. The
locus you have is on the card given to you. Each person in your pair will roll one die.
The roll of the dice will determine your alleles for your loci. Because there are six sides
to the dice, you will have six different allele possibilities (1, 2, 3…6). You will record
your findings on your Fictitious DNA Profile worksheet and on the class worksheet
(project the class results). For example, if my partner rolled a 5 and I rolled a 2 for the
TPOX locus, then we would write 5 and 2 in Identified Alleles section of the worksheet.
We would write 1/6, 1/6 for the Allele Frequency and 2(1/6 x 1/6) for the Formula. Then
we would calculate the Locus Frequency. 2(1/6 x 1/6) = 1/18. Explain the formula is
(1/6 x 1/6) when you roll double because order doesn’t matter. For instance rolling a 3, 3
is exactly the same as rolling a 3, 3. There is only one way, not two to get this result.
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Students create a fictitious DNA profile: Students roll dice and fill out the class profile.
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Explain Fictitious DNA Profile activity: Look at the alleles that we had for the TPOX
locus. What is the probability of getting a 5, 2? 1/18. For next period to come in and
roll the dice, what is the probability that they would roll a 5, 2? 1/18. What if everyone
rolled for the TPOX loci? What is the chance that a student next period would get the
same result? Number of students x 1/18. How many people are there in the world?
6,708,718,217. How many people could have a 5, 2 according to our model?
6,708,718,217 x 1/18 = 372,706,567. So there is 372,706,567 people in the world with
that same allele combination. Do you think that is enough evidence to convict
someone—to send them away to prison for their entire life? What if you test the next loci
and the result is 3,3? 372,706,567 x 1/36 = 10,352,960. Do you think that is enough
evidence to convict someone? What if you test the next loci?
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Students calculate a combined genotype frequency: Students work in pairs to calculate
the genotype frequency for their fictitious DNA profile. Then students write their
answers in the space provided on the Fictitious DNA Profile activity.
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Share out results and discuss: The students share out their calculations for the combined
genotype frequency. By looking at only eight loci, we can be sure that it is this person
and only this person (except in the case of identical twins or clones). You can’t have
0.05 of a person, which is a rough probability or having another person’s DNA match,
Developed by Jesse Wade and Amy Callahan
High Tech High, San Diego, CA
July 8, 2008
once eight loci are tested. In the U.S., we extend this to 13 loci, making another match
statistically impossible. We, however, are not like dice. We don’t have six possible
alleles for every loci. Each loci has a different number of alleles and you have a different
probability of having each allele. I will show you an article from the Journal of Forensic
Science that shows the probability of having each allele on 15 STR loci for U.S.
Caucasions, African Americans, and Hispanic Populations. Please skim the article
individually and then discuss in your group the Table Talk questions. Have one person
from each group take notes on your discussion.
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Pass out materials: Pass out one Journal of Forensic Science article to each student.
Then pass out one Table Talk per student group.
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Students discuss the Table Talk questions.
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Bridge to next lesson: Teacher asks students to share out what they talked about in the
group. Teacher sums up answers to the questions: 1) The more loci you test, the surer
you are you have a match. This is due to the fact that each loci tested is independent of
the other, and when we multiply the probabilities of independent events occurring, the
end result is a very small probability. 2) The people could be identical twins or clones.
3) Descriptions of a suspect, descriptions of a vehicle, shoe prints and clothing can help
narrow down potential suspects as well. For example, if the suspect is a male, divide the
population in half. If the suspect is red haired multiply the remaining suspect pool by
1/20. If the suspect is 6’2” multiply the remaining suspect pool by 1/25… 4) Allele
frequencies between populations are different. 5) You may not know the race of the
person being tested, or they might incorrectly identify race. Race is often a social
construct and not biological. Ask students what questions they have now after the lesson.
Remind students that will be using this probability in the future.
ASSESSMENT: Assessments are formative for this lesson. The teacher monitors student work
on the Fictitious DNA Profile activity and classroom conversations. The teacher reads the table
talk notes. Students will have a formal assessment in which they calculate the DNA probability
on a case independently later in the unit.
REFERENCES:
Access Excellence @ The National Health Museum (2008). Understanding Gene Testing.
Retrieved July 8, 2008 from
http://www.accessexcellence.org/AE/AEPC/NIH/gene03.php
Bio-Rad. (2007). Biotechnology Explorer: Crime Scene Investigator PCR Basics Kit. Retrieved
on July 8, 2008 from http://www.explorer.bio-rad.com
Butler, J. M., Schoske, R., Vallone, P.M., Redman, J.W., & Kline, M.C (2003). Allele
Frequencies for 15 Autosomal STR Loci on U.S. Caucasian, African American, and
Hispanic Populations. Journal of Forensic Science. 48(4). pp. 908-911. Retrieved on
July 8, 2008 from http://www.cstl.nist.gov/div831/strbase/pub_pres/Butler2003a.pdf
Developed by Jesse Wade and Amy Callahan
High Tech High, San Diego, CA
July 8, 2008
Butler, J. M. & Reeder, D. J. (June 23, 2008). FBI CODIS Core STR Loci: 13 CODIS Core STR
Loci with Chromosomal Positions. Retrieved July 8, 2008 from NIST Standard
Reference Database: http://www.cstl.nist.gov/biotech/strbase/fbicore.htm
Developed by Jesse Wade and Amy Callahan
High Tech High, San Diego, CA
July 8, 2008