Interpretation of a Single Source Unmixed Profile

Question 1

What can be said about a 17 locus DNA profile which is complete, unrelated and unmixed?

However, what can reduce the statistics associated with full 17 loci matches?

Answer 1

• exceedingly rare so the chance of a coincidental match between two randomly chosen set of individuals is incredibly small
• mixtures
• incomplete profiles
• relatives

Question 2

What is the expectation for any DNA profile match?

What is this supported by?

Answer 2

• for a statistic to be attached
• need to generate a specific weight to that DNA profile match when providing it in evidence
• supported by Forensic Science Regulator

Question 3

What things complicate attaching a statistical weight to DNA profiles?

Answer 3

• where they are mixtures, include relatives or where the info in the sample is incomplete

Question 4

What is the question we ask in relation to the statistics of a DNA profile?

Answer 4

• what is the chance that a randomly chosen, unrelated person would have the same DNA profile that we are seeing in at the evidence sample?

Question 5

What are the two types of samples and what are the quality issues associated with both?

Answer 5

• reference sample (not expected to be a mixture)
• crime sample (not uncommon to have mixed or degraded materials)

Question 6

What does the allele count at a particular locus represent?

Answer 6

• represents the number of base pair repeats at that locus (where loci represent location on particular chromosome)

Question 7

What are the two questions to deal with in a profiling test?

Answer 7

• who might be included OR excluded as a possible contributor to that evidence sample

Question 8

What happens when exclude possible contributor

Answer 8

• no statistical weight as exclusion is absolute

Question 9

What are the two assumptions to the product rule?

What if this is absent?

Answer 9

• independence within loci (presence of 12 and 15 allele at D8 locus is independent of each other)
• independence across loci (presence of alleles is also independent across loci)
• if independence is absent then unable to apply product rule

Question 10

What is the Hardy-Weinberg equilibrium?

What does this principle underlie?

Answer 10

• P^2 + 2pq + q^2 = 1
• this principle underlies the product rule

Question 11

What is the process of calculating the match probability?

Answer 11

• calculate LOCUS FREQUENCY using allele frequency databases
• for homozygous loci use P^2
• for heterozygous loci use 2PQ
• calculate PROFILE FREQUENCY
• the product of all the loci frequencies
• calculate MATCH PROBABILITY
• 1 divided by profile frequency

Question 12

What are allele frequency databases?

Answer 12

• database of allele frequencies that represent allele frequencies among the population
• UK: undertook referencing sample from wide range of population. began in West Midlands
• USA: FBI took DNA samples from 200 recruits in 1980

Question 13

What does a 14.5 % allele frequency of 12 allele at D8 locus mean?

Answer 13

• 14.5 % is frequency of 12 allele at D8 locus in Caucasian population

Question 14

What is the match probability in the UK?

What principal is used to calculate this?

Answer 14

• it is 1 in 1 billion
• using the profile ceiling principal

Question 15

What can be applied as a simple correction for sampling bias

Answer 15

• balding correction for size bias

Question 16

the concept of DNA interpretation finds its roots in what genetics?

Answer 16

• population genetics

Question 17

How do you present statistics?

Answer 17

• the chance of another randomly selected, unrelated individual, in
  the given population (Caucasian) having an identical DNA 17 profile that is seen in the crime scene
  sample is 1 in (match probability)

Question 18

during the Pitchfork case, Jeffreys used a type of repeating (DNA) unit different to the ones used today. The repeated segments, in this case dozens or even 100 bases long, can be observed. These are referred to as?

Answer 18

• minisatellites