Science | Laboratory Instructions | DNA
Laboratory Instructions
Laboratory: DNA
Materials
Supplied
Student Guide
Laboratory Guidelines
Electropherograms (online)
Codis STR Table (online and print)
Also Needed
scientific calculator (optional)
Safety
Review the Laboratory Guidelines before conducting the lab.
Crime Summary
On a Midwest college campus, three female students were assaulted within a six-month period. Each woman was
beaten, robbed, and left for dead. All three women survived the attacks. During the last attack, the perpetrator
dropped a bloodstained knife, which was recovered by police.
When questioned by police, each woman said that the attacker was a tall man who wore gloves and a mask. He
threatened them with a knife and took them off the main campus quad into a wooded area where he beat them.
Although they could not see the attacker’s face, each woman did manage to scratch him. Police investigators took
scrapings from under their fingernails, blood samples, and their bloodstained clothes for DNA analysis.
In the course of the investigation, police detained four possible suspects. All the suspects are white males.
Although Suspects A, B, and C could not be held because the victims could not identify them from a lineup, police
did manage to get saliva samples. Unbeknownst to the suspects, police collected discarded cigarette butts and
drinking glasses so the saliva could be used for DNA testing.
Suspect D was arrested shortly after the last assault. He had scratch marks on his arms, and police confiscated a
bloody pair of jeans from his apartment. They took DNA samples from him and the clothing.
They have sent the evidence to you, the forensic scientist, to conduct the analysis.
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Page 1 of 4
Science | Laboratory Instructions | DNA
Part 1: Use an electropherogram to create a DNA profile.
You have run the PCR with STRs for all of the evidence samples and suspects. You have used the following
STRs:

Amelogenin

D3S1358

vWA

FGA

D18S51
You have separated the PCR product DNA fragments by capillary electrophoresis and have obtained the
electropherograms corresponding to each sample.
Go online to locate the electropherograms. They are organized across five screens into the following sets:

DNA from Victims

DNA from Victims’ Shirts

DNA from Victims’ Fingernail Scrapings

DNA from Knife and Jeans

DNA from Suspects
Study the electropherogram for Victim 1. The first thing you will notice is that the number of STRs (allele repeat
number) at each marker is not available. Only the base pair length is available.
Computers generally calculate the number of STRs from the base pair length, but in this lab, you will do the work
of the computer. Let’s walk through the process.
1. For each STR marker, locate the base pair length (bp on the horizontal axis) for the peak or peaks. For
example, for Victim 1, there is one peak for the STR marker D18S51. The peak is located at bp 272.
2. Now use the CODIS STR Marker table. Click Codis STR Table in the upper right-hand corner of the
electropherogram to access the table online. You can also use the print version of the table. To convert
the bp number to the allele repeat number:

Locate the base pair length in the appropriate STR marker column.

Read the allele repeat number in the same row.
For example, the D18S51 peak for victim 1 has a bp value of 272. Find the row with 272 under the
D18S51 column. Then read the value in the same row under the Allele (repeat #) column, which in this
case is 9.
Example: Allele repeat number for Victim 1, D18S51
Base pair length (bp)
Allele (repeat #)
Amel
D3S1358
7
8
99
103
D18S51
264
268
9
107
272
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vWA
FGA
Page 2 of 4
Science | Laboratory Instructions | DNA
Note that Victim 1 is homozygous for D18S51 (one peak). That is, if you find one peak for STR (D18S51), then
you know she inherited STR D18S51 (with an allele repeat number of 9) from BOTH parents. This is recorded as
(9, 9) in the DNA profile. If there are two peaks nearby each other for STR, then you know she is heterozygous for
this STR, and you need to locate the Allele repeat number for each peak. Notice below that FGA, for example,
has two Allele repeat numbers, one for each peak on the electropherogram.
Your job now is to complete the DNA Profile Results: Table of Alleles below. The DNA profile of Victim 1 for all
five STR markers has been recorded for you. Study the values and the electropherogram for Victim 1 to make
sure you understand the process. When you have finished the table, go to the next part of the lab.
DNA Profile Results: Table of Alleles
Sample
Base pair length (bp)
Amel (XX or XY)
D3S1358
vWA
FGA
D18S31
XX
11, 12
15, 15
13, 14
9, 9
Victim 1
Victim 2
Victim 3
Victim 1 shirt
Victim 2 shirt
Victim 3 shirt
Knife
Suspect D jeans
Victim 1 fingernail scrapings
Victim 2 fingernail scrapings
Victim 3 fingernail scrapings
Suspect A
Suspect B
Suspect C
Suspect D
Part 2: Draw conclusions.
Examine the DNA profile results table.
1. Which suspects can you exclude?
2. Which suspects can you include?
3. Can you associate any of the suspects with specific victims? If so, which combinations?
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Page 3 of 4
Science | Laboratory Instructions | DNA
4. Are there any inconsistencies?
Part 3: Search CODIS database.
Compare the DNA profiles from this case to those in the CODIS database below. Sample codes that start with E
stand for evidence, while those that start with O stand for offenders or suspects who were arrested. Do any of the
DNA profiles from your case match any in the CODIS database? If so, which match occurs, and what is the
CODIS case?
CODIS Database
Sample
Amel
E00100009
E00106010
E00260050
E03010025
E00562110
O00106530
O00352130
O01106220
O01897830
O05534297
Sample
E00100009
E00106010
E00260050
E03010025
Vaginal swab,
unsolve
E00562110
O00106530
O00352130
O01106220
O01897830
O05534297
D3S1358
XY
XX
XY
XY
XY
XX
XY
XY
XY
XY
16, 17
12, 13
14, 14
14, 14
19, 20
18, 18
15, 15
17, 17
13, 14
20, 20
vWA
14, 15
19, 19
18, 19
16, 16
13, 14
14, 15
12, 12
15, 15
20, 20
18, 19
FGA
25, 25
13, 14
24, 24
20, 21
22, 23
19, 19
17, 18
24, 25
22, 23
20, 21
D18S51
10, 11
12, 12
7, 7
10, 10
15, 15
11, 11
16, 16
15, 16
12, 13
7, 8
Description
Bloodstain, unsolved murder in Pittsburgh, PA
Bloodstain, unsolved murder in Washington, DC
Oral swab, unsolved sexual assault in Tampa, FL
Vaginal swab, unsolved rape case in Yuma, AZ
Vaginal swab, unsolved rape case in Los Angeles, CA
Sexual offender in Chicago, IL
Robbery suspect in Raleigh, NC
Burglary suspect in Houston, TX
Sexual offender in New Orleans, LA
Vehicular manslaughter suspect in Seattle, WA
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Copying or distributing without K12’s written consent is prohibited.
Page 4 of 4