The Case of the Druid Dracula by Peggy Brickman

Part III—More Analysis

The crime was featured on BBC’s Crimewatch program in December 2001 and North Wales Police received over 200 calls. Following up reports of a teenager who had attacked a German student, the police went to the home of Matthew Hardman (suspect 1), who gave police a cheek swab. During this visit, officers found a pair of Levi shoes. Forensic Science Service (FSS) scientists matched Hardman’s shoes to the footwear marks found at the murder scene. Profiling suggested a much older offender, so another suspect was also asked to give a cheek swab (suspect 2). Since both suspects were men, the officers needed to test for other genetic differences. They focused on STRs (short tandem repeats), stretches of DNA that exist in all people, but in different numbers of repeats. The allele ladder below shows all varieties in a population.

Electrophoresis

Questions

  1. Which suspect presents a profile compatible to the sample found at the crime scene?
    1. Suspect #1
    2. Suspect #2
  2. There are only a few different numbers of repeats that are seen in our population—only 5 different TPOX STRs for example. By testing thousands of DNA samples, researchers know the distribution of these different STRs in the general population; those allele frequencies are shown in the table below. Using these frequencies, one can determine the probability that someone else at random would have the same matching pattern. For example, what is the likelihood that someone else at random would have the same pattern of Matthew Hardman (a 5 and 7 repeat for the THO1 STR)?
    THO1TPOXCSF1PO
    5: 1/2008: 1/29: 1/40
    6: 1/49: 1/810: 1/5
    7: 1/610: 1/1811: 1/3
    8: 1/711: 1/512: 1/3
    9: 1/612: 1/2013: 1/10
    9.3: 1/3 14: 1/50
    10: 1/100  
    1. 1/200
    2. 1/206
    3. 1/1200
    4. 1/2600
    5. 1/20060
  3. What is the probability that someone else at random would have that same pattern of THO1 5 & 7, TPOX 8 & 9, and CSF1PO 11 & 12?
    1. 1/1600
    2. 1/7200
    3. 1/17600
    4. 1/172800
    5. 1/1555200

Originally published at http://www.sciencecases.org/druid_dracula/druid_dracula3.asp

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