Name: CODIS STRs and Probabilities
While a lot of DNA contains information for a certain function, there is some called junk DNA, which is currently used for human identification. At some special locations (called loci) in the junk DNA, predictable inheritance patterns were found to be useful in determining biological relationships.
These locations contain specific DNA markers that DNA scientists use to identify individuals. In a routine DNA paternity test, the markers used are
Short Tandem Repeats (STRs), short pieces of DNA that occur in highly differential repeat patterns among individuals.
Each person’s DNA contains two copies of these markers—one copy inherited from the father and one from the mother. Within a population, the markers at each person’s DNA location could differ in length and sometimes sequence, depending on the markers inherited from the parents.
The combination of marker sizes found in each person makes up his/her unique genetic profile. When determining the relationship between two individuals, their genetic profiles are compared to see if they share the same inheritance patterns at a statistically conclusive rate.
For example, the following sample report from this commercial DNA paternity testing laboratory Universal Genetics signifies how relatedness between parents and child is identified on those special markers:
DNA Marker
D21S11
D7S820
TH01
D13S317
D19S433
Mother
28, 30
9, 10
14, 15
7, 8
14, 16.2
Child
28, 31
10, 11
14, 16
7, 9
14, 15
29, 31
11, 12
15, 16
8, 9
Alleged father
15, 17

Name: CODIS STRs and Probabilities
The partial results indicate that the child and the alleged father’s DNA match among these five markers. The complete test results show this correlation on
16 markers between the child and the tested man to draw a conclusion of whether or not the man is the biological father.
The Federal Bureau of Investigation (FBI) of the US has been a leader in developing DNA typing technology for use in the identification of perpetrators of violent crime. In 1997, the FBI announced the selection of 13 STR loci to constitute the core of the United States national database, CODIS. All CODIS
STRs are tetrameric repeat sequences. All forensic laboratories that use the
CODIS system can contribute to a national database. DNA analysts like Bob
Blackett can also attempt to match the DNA profile of crime scene evidence to
DNA profiles already in the database.
There are many advantages to the CODIS STR system:
The CODIS system has been widely adopted by forensic DNA analysts
STR alleles can be rapidly determined using commercially available kits.
STR alleles are discrete, and behave according to known principles of population genetics
The data are digital, and therefore ideally suited for computer databases
Laboratories worldwide are contributing to the analysis of STR allele frequency in different human populations
STR profiles can be determined with very small amounts of DNA
As part of his training and proficiency testing for DNA Profile analysis of STR
(Short Tandem Repeat) Polymorphisms, Forensic Scientist and DNA Analyst
Bob Blackett created a DNA profile on his own DNA. Here is Bob's DNA Profile for the 13 core Genetic Loci of the United States national database, CODIS

Name: CODIS STRs and Probabilities
(Combined DNA Index System):
Locus
D3S13
58 vWA FGA D8S1179 D21S11 D18S51 D5S818
Genotype 15, 18 16, 16 19, 24 12, 13 29, 31 12, 13 11, 13
Frequency 8.2% 4.4% 1.7% 9.9% 2.3% 4.3% 13%
Locus
D13S3
17
D7S820
D16S53
9
THO1 TPOX CSF1PO AMEL
Genotype 11, 11 10, 10 11, 11 9, 9.3 8, 8 11, 11 X Y
Frequency 1.2% 6.3% 9.5% 9.6% 3.52% 7.2% (Male)
For each genetic locus, Bob has determined his "genotype", and the expected frequency of his genotype at each locus in a representative population sample.
For example, at the genetic locus known as "D3S1358", Bob has the genotype of
"15, 18". This genotype is shared by about 8.2% of the population. By combining the frequency information for all 13 CODIS loci, Bob can calculate that the frequency of his profile would be 1 in 7.7 quadrillion Caucasians (1 in 7.7 times
10 to the 15th power!
In Bob's forensic DNA work, he often compares the DNA profile of biological evidence from a crime scene with a known reference sample from a victim or suspect. If any two samples have matching genotypes at all 13 CODIS loci, it is a virtual certainty that the two DNA samples came from the same individual (or an identical twin).

Name: CODIS STRs and Probabilities
Using Table 2 p. 261 in your book, calculate the following probability problems:
For example: If only 2 STR loci were used, D3S1358 and VWA, the probability that 2 American Caucasian individuals selected at random would be the same is one in 185.2 people:
Multiply 0.080 x 0.068 = .0054
Solve for X X= 1/.0054 = 185.2 people
Probability= 1/ 185.2 people
Problem #1: (5 pts)
What is the likelihood that 2 African Americans chosen at random are the same if you only used TPOX, CFS1PO, D19S51?
Problem #2: (5 pts)
What is the likelihood that 2 American Caucasians selected at random would match for all 13 loci?

Name: CODIS STRs and Probabilities
Locus
D8S1179
D21S11
D7S820
CSF1PO
D3S1358
TH01
Problem #3
Case Study: (15 pts)
A 21 year old woman with severe physical and mental disabilities was taken to the emergency room with genital bleeding. From a spindle shaped blood clot, microscopic examination showed a small number of chorionic villi (remnants of a placenta). On the suspicion that the pregnancy was the result of abuse, the case was reported to the public prosecutor, who ordered DNA profile analysis. The data in Table 1 were collected.
Who do you think impregnated the woman to produce the fetal tissue? Explain why you came to your conclusion as if you were explaining it in court to a jury. (Assume you need to explain your
chain of logic).
Table 1-STR profiles of the victim, fetal tissue, and victim’s male relatives
Victim Fetal
Tissue
15
28, 29
10
11, 12
15
Victim’s
Brother
15
29 ,30 30
10
11
10, 12
Victim’s
Father
Victim’s
Paternal grandfather
13 ,15 13, 15
29, 30
8,10
10, 11 11
27 ,29
8
11
Victim’s
Maternal grandfather
10, 13
28, 31.2
10, 12
10, 12
16, 18
6,8
D13S317
D16S539
D2S1338
D19S433
15
8,9
15
8
16, 16.2 14,
16.2
15
8,9
15
9
15, 16
6 ,9
11, 12
9, 22
17, 20
12 12,13 11 ,13 9, 11
9, 14 9, 14
17, 18 18, 20
11, 14
20
11
20, 24
14, 16.2 14, 16 12, 14
12
9, 12
18, 20
13, 15
VWA
TPOX
D18S51
D5S818
FGA
14, 17
8, 9
AMELOGENIN X, X
20, 24
17 17, 18 14, 18 14, 17
8, 11 8, 11
20, 21 21, 24
8, 9
20, 21
9, 10
10, 22 10, 15 10, 15 15, 22 14, 15
13
X, X
13
X, Y
13
X, Y
13
X, Y
11, 13
21
17, 19
8, 11
13, 14
X, Y
13
21, 23

Name: CODIS STRs and Probabilities