STR profile analysis - DNA databases & informative stats

Question 1

What is the aim of DNA profiling and what does it require

Answer 1

to link a sample to a crime scene
- Requires DNA from a crime scene and a suspect for comparison

Question 2

If a crime scene sample matches with a sample, from a known reference sample or one found on the UK National DNA Database, what has to be done

Answer 2

The statistical likelihood of that match occurring by chance needs to be calculated from match probabilities using an allele frequency database

Question 3

What year was the UK National DNA Database established and why did it take so long

Answer 3

1995 - early techniques proved incompatible with data basing

Question 4

How did the UK National DNA Database come about

Answer 4

1. Standardisation between Forensic practitioners to reduce ambiguity
2. Formation of European DNA profiling group - adopted standards and common approaches
3. Met Police established an in-house database and discovered the need for a national one
4. Increase in funding for forensics

Question 5

What does the UK National DNA Database contain

Answer 5

Individual samples DNA STR profiles - not the physical DNA

Question 6

What can the UK National DNA Database produce

Answer 6

1. Full matches
2. Partial matches - due to poor amplification

Question 7

What is the purpose of a staff elimination database

Answer 7

to preserve the integrity of the national DNA database by preventing the addition of DNA profile derived as a result of contamination

Question 8

Name 3 ways contamination events have effected cases

Answer 8

1. Mislead high-profile police investigations
2. Wasted resources associated with significant costs
3. Delayed cases reaching judicial conclusions

Question 9

Who is included within a staff elimination database

Answer 9

• All scene-going staff
• All personnel involved in seizure of exhibits
• Evidence-related property officers
• Custody officers
• All personnel involved in handling unpackaged exhibits

Question 10

Is providing your DNA for a staff elimination database mandatory or voluntary in the UK

Answer 10

Mandatory

Question 11

Why do we conduct statistical analysis of inclusive STR profile results

Answer 11

to ensure validity

Question 12

What do we use allele frequency databases for

Answer 12

to calculate match probabilities

Question 13

How many allele frequency databases do we have in the UK

Answer 13

5 - separated by race

Question 14

in terms of match probabilities what formula do we use for homozygotes compared to for heterozygotes

Answer 14

homozygote = p^2
heterozygote = 2pq

Question 15

How do we calculate the likelihood ratio from a match probability

Answer 15

1/ match probability

Question 16

How do we calculate the match probability of an entire STR profile

Answer 16

by multiplying each individual locus match probability value together

Question 17

If the likelihood ratio value is extremely large, what do we use instead and why

Answer 17

1 in a billion - to reduce confusion of the jury

Question 18

What is the only problem with using p^2 and 2pq equations for match probabilities

Answer 18

they do not account for population structure

Question 19

How does using an allele frequency database not account for population structure

Answer 19

just because you know the ethnic group of the suspect, does not mean you know the ethnic group of the perpetrator

Question 20

How do we account for population structure within our match probability calculations

Answer 20

by using a fixation index (Fst)

Question 21

What is the range of Fst values for humans

Answer 21

0.01-0.05