miniPCR bio™ Electrophoresis Forensics Lab. Wrongfully Convicted? Instructor's and Student's Guide
Version: 1.1 - Release February 2022 - © 2022 by miniPCR bio™
Student's Guide
P./11
STR profile
STR
location
STR 1 5, 8
12, 13
11, 12
12, 12
12, 18 12, 18 12, 18
12, 12 12, 12
11, 12 12, 12
12, 13 14, 14
5, 8 5, 8
STR 2
STR 3
STR 4
STR 5
Evidence
genotype
Suspect 1
genotype
Suspect 2
genotype
Result Match No match
In forensic investigations, scientists examine
if a person's STR genotypes match the DNA
evidence. Comparing the number of repeats
at multiple locations in the genome can create
a unique DNA profile for genetic identification
(Figure 3). The more STR regions compared
in a forensic investigation, the more likely it
is to find differences between people's STR
profiles.
Police can compare STR profiles from DNA
evidence with DNA from a suspect if they
have one. But law enforcement agencies
can also compare DNA evidence to databases
of DNA profiles. The FBI's Combined DNA
Index System (CODIS) is a DNA profile
database for 20 standard STR locations
(Figure 4). As of 2020, CODIS contains more
than 19,000,000 DNA profiles from convicted
offenders and individuals who have been
arrested, as well as other DNA samples
from forensic investigations (FBI, 2020). A
centralized DNA database means that local,
state, and federal authorities can all compare
DNA profiles in the same way. And as of 2020,
close to 500,000 criminal investigations have
been aided by CODIS in the US (FBI, 2020).
Figure 3. Example STR profiles
Forensic scientists compare STR genotypes at multiple
locations with variable STRs to determine whether DNA
samples could have come from the same person. In this
example, the STR genotypes show that suspect 1 could
have been the source of the DNA evidence, but suspect
2's DNA does not match the evidence.
Figure 4. STR locations
Asterisks indicate the chromosomal locations of the 20
standard STR locations used in the FBI CODIS system.
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 X Y
FBI. "CODIS - NDIS Statistics." September 2020.
https://www.fbi.gov/services/laboratory/biometric-analysis/codis/ndis-statistics. Accessed 11/20/2020.
