Question

12. About midnight on Saturday, the strangled body of a regular patron of the Seedy Lounge is found in an alleyway near the bar. The police interview the workers and patrons A-E remaining in the bar. A few of the patrons indicate that several individuals, including the bartender, owed money to the victim. The police notice that the bartender and patrons have recent cuts and scratches on their faces and backs of their necks, but are told that these happened during mud-wrestling matches earlier in the evening. DNA samples are obtained from the bartender and the bar's patrons, from tissues of the victim, and from scrapings of her fingernails. Microsatellite analyses are performed on the DNA samples using three microsatellites, CSFIPO, THO1, and D18S51. The number of microsatellite repeating units in each of the DNA samples is identified by PCR and gel electrophoresis, and are given in the table below. | | DNA sample Victim Victim's fingernail scraping Patron A Patron B Patron C Patron D Patron E Bartender Microsatellite CSF1P0 THÓI | 9,12 7 6, 9,12,16 5, 6, 7,9 9,16 5, 10 15, 16 5,9 13 8 4, 12,16 9 5. 10, 125, 6 9. 11 8 , 11 D18S51 1 4,16 13, 14, 16 1 4, 16 12 1 2, 15 1 3, 14 12 1 3,16 A. What is the total number of microsatellite alleles are present for CSF1 PO in these individuals? (2 pt) B. How do you explain the appearance of only one allele in some individuals? (2 pt) 9o

Answer 1

A)

There are 7 alleles for the CSF1PO STR locus. These are 9, 10, 11, 12, 13, 15 and 16.

B)

Some individuals carry only one allele for a given STR locus (For example, for the CSF1PO locus, Patron C only has the 13 allele) as they are Homozygous for that allele. This means both chromosomes in that individual, the maternally inherited chromosome and the paternally inherited chromosome, both carry the CSF1PO-13 allele.

C)

Three or four alleles appear in the DNA samples obtained from the victim's fingernails as the victim has one or two alleles of their own, while also having the DNA of the people they came in contact with (possible perpetrators). As fingernails would have both the attacker's and the victim's DNA, which may be different, the DNA extracted from the Fingernails can have 3-4 alleles.

D)

Based on results obtained from the CSF1PO marker alone, police should investigate Patron A and Patron D. This is because the victim's fingernails have the 9, 12 and 16 alleles for the CSF1PO marker and while they have the 9, 12 alleles themselves, their attacker must have left the 16 allele.

This is possible for Patron A, Patron B, and Patron D. However, Patron B also have the 15 allele at this locus, which would have shown up in a DNA profile, therefore, they can be excluded.

E)

Based on the results obtained from all three markers, police should investigate Patron D. Since it is possible for both Patron A and Patron D to be the attacker, the TH01 marker rules out Patron A. For this locus, Patron A has the 5 and the 10 allele. However, the 10 allele for TH01 was not recovered from the Victim's fingernails.

However, the STR profile of Patron D matches with that obtained from the Victim's fingernails for all three loci, and they cannot be ruled out. Therefore, the police should investigate Patron D.