Week 11 - Forensic Science

Question 1

Stages of DNA Profiling

Answer 1

1. DNA extraction
   - manual/automated
   - inhibitory substances common
   - complex substrates
2. DNA quantitation
   - real time PCR
   - human-specific
3. Amplification
   - multiplex PCR
4. Analysis
   - capillary electrophoresis

Question 2

Desirable Features for Forensic Short Tandem Repeats (STR) Systems

Answer 2

High heterozygosity (differentiate people)
Regular repeat unit (predictable alleles)
Distinguishable alleles - we mostly analyse tetra-nucleotide (4 bp) repeats (CE technology)
Robust amplification (inhibitors)
Can be multiplexed (less DNA required, faster)
Small product sizes (<500 bp range) are better for degraded DNA

Question 3

What Can be Determined from a Forensic DNA Profile?

Answer 3

Only gender

Question 4

What Can’t be Determined from a Forensic DNA Profile?

Answer 4

Ethnicity, racial background, country of birth or skin colour
Age of the person
When the person is going to die
Physical appearance (hair colour, eye colour, height, etc)
Sexual orientation.
Past medical history (e.g. Venereal disease)
Current medical conditions (e.g. Diabetes)
Predisposition to future medical conditions (e.g. Cancer)

Question 5

Forensic Science - Positive Results

Answer 5

The current methods are VERY discriminating
The chance of two people (apart from identical twins) having the same full DNA profile is VERY remote
Not everyone has been profiled, so the statistics help us to determine how likely it is that two people would have the same profile
The statistical result we give is NOT the probability of guilt
Statistically, we assume it is someone else

Question 6

Forensic Science - Negative Results

Answer 6

We can ONLY comment on the laboratory results, not on whether or not something has actually happened.
A negative laboratory finding does not mean that something did or did not happen
Spermatozoa are not detected in ~50% of all sex assault cases

Question 7

Familial Database Searching

Answer 7

Premise: A DNA profile has been recovered from a serious offence
There is no matching DNA profile on the state or national DNA databases
While the unknown offender is not on the DNA database, is it possible that a close biological relative is?
Primarily looking for potential:
- sibling relationships (overall number of matching alleles)
- parent / Child relationships (minimum of one shared allele at each locus)

Question 8

Markers used for Forensics - Tests with Fast Speed of Analysis but with Low Power of Discrimination

Answer 8

ABO blood groups
Single STR
Polymarker D1S80

Question 9

Markers used for Forensics - Tests with Slow Speed of Analysis but with High Power of Discrimination

Answer 9

RFLP Multi-locus probes
RFLP Single-locus probes

Question 10

Markers used for Forensics - Tests with Fast Speed of Analysis but with High Power of Discrimination

Answer 10

Multiplex STRs