PBL3: DNA Forensics
Ralph Henderson, age 43, was found dead by four birdwatchers in late October in Starved
Rock State Park. The body was a few feet from a side road in a clump of brush. (See
maps). The county coroner determined that Ralph had been dead for about ten days, the
cause of death was five gunshot wounds to the chest, and his body had been moved after
he was killed.
Ralph had resided his entire life in nearby Ottawa, Illinois, where he lived alone in an
apartment. Several of the other tenants knew him well, and he had a number of personal
friends. None of these people had seen him for a few days, but this was not unusual
because he frequently traveled to Chicago or Las Vegas. Ralph had inherited a modest
fortune and a substantial interest in a wholesale furniture business that had been founded
by his great-grandfather; however, he took no part in its management.
Ralph had three cousins with whom he was not personally close and who would benefit
financially from his death. He and his estranged wife, Velma, maintained a stormy
relationship and were thought by many to despise each other. Velma said that he owed a
large sum of money to a bookie, but no one else was aware of this, and no corroborating
evidence could be found. The murder weapon was assumed to be on the bottom of the
adjacent Illinois River, but it was not found there or elsewhere. Velma said her last
contact with Ralph had been three weeks before his body had been found. She was the
prime suspect, but neither her observed pattern of behavior nor any available physical
evidence tied her to the crime.
During the second week in January, based upon a report from an observant local
automobile mechanic, law enforcement authorities obtained a search warrant and
examined Velma's car. They found faint stains, which soon were identified as human
blood, in the trunk. Velma said it was the blood of a helpful stranger who had assisted her
several months before when she had a flat tire along Interstate 80. He had cut himself
while changing the tire, and while the cut was not serious, it had bled freely. It was
confirmed that Velma had a tire repaired in early September, but the stranger could not be
located.
A blood sample from the car, along with a sample of Ralph's blood, was sent for analysis
using the FBI's CODIS system. CODIS (Combined DNA Index System) uses genetic
polymorphisms at 13 STR (Short Tandem Repeat) loci. Each of the two samples gave the
following identical result.
LOCUS
ALLELES
D3S1358
14
17
vWA
17
17
FGA
21
22
D8S1179
13
13
D21S11
29
33.2
D18S51
14
14
D5S818
12
12
D13S317
11
13
D7S820
9
10
D16S539
12
14
THO1
6
7
TPOX
8
11
CSF1PO
12
12
Questions
1. What is the probability that these two blood samples are from two random
individuals in the population? [The Canadian Society of Forensic Science
homepage (www.csfs.ca) is a good source of STR data bases, and also provides a
random match probability calculator.]
2. Ralph was white and Velma told the authorities that the helpful stranger was also.
Would your answer be different if the stranger were black?
3. How would a random match probability calculator be constructed? Are there
issues regarding the numerical calculations that should be considered? Would it
be difficult to implement the calculator in a spreadsheet?
4. Did Velma kill Ralph?
5. The ability to test DNA types leads to various ethical or legal questions. Discuss
one ethical issue related to DNA typing.
6. See http://www.cstl.nist.gov/biotech/strbase/index.htm for more information,
including the nucleotide sequences, more population data, and a Powerpoint
overview of how STS is used in forensics.