Single source DNA profiles

Question 1

What do we need to do with a DNA profile?

Answer 1

• There is an expectation that for any DNA profile match a statistic be must be attached
• We need to generate a specific weight to that DNA profile match when providing this in evidence.
• This is complicated by mixed profiles, incomplete information and relatives.

Question 1

1. The statistics associated with full 17 loci matches will be reduced in cases where we encounter incomplete profiles, relatives and what else?
2. What is missing from this sentence? What is the chance that a ———— ———- unrelated person would have the same DNA profile that we are seeing in the evidence sample?
3. In contrast to the reference sample, we might expect to see a mixture in the ——— sample
4. The allele count at a particular locus represents the number of repeating?
5. Where only one peak is seen on the EPG, this is said to be?
6. 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 1

1. The statistics associated with full 17 loci matches will be reduced in cases where we encounter incomplete profiles, relatives and mixtures.
2. What is missing from this sentence? What is the chance that a randomly chosed unrelated person would have the same DNA profile that we are seeing in the evidence sample?
3. In contrast to the reference sample, we might expect to see a mixture in the crime sample.
4. The allele count at a particular locus represents the number of repeating base pairs
5. Where only one peak is seen on the EPG, this is said to be homozygous.
6. 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 minisatellites.

Question 2

Statistical weight when excluding someone for a DNA profile.

Answer 2

• We do not have do add a statistical weight to an exclusion
• The exclusion is absolute
• The person is excluded as a contributor to the evidence sample.

Question 3

What wording should be used when applying statistical weighting for an evidence sample that has the same profile as a reference sample and the source of the reference cant be excluded as the source of the evidence sample?

Answer 3

What is the chance that a randomly chosen, unrelated person might happen to have the same DNA profile that we find associated with an evidence sample?

Question 4

Assumptions for the product rule

Answer 4

• The first of these assumptions is the events that we are evaluating are independent of each other. (The presence of one allele does not affect the likelihood of the presence of another allele)
• This applies to across the loci and wihtin the locus.
• If this assumption isn’t true, that is to say that there is the absence of independence of alleles within and between loci, then we are unable to apply the product rule and multiply frequencies together.

Question 5

1. When comparing EPGs we compare the evidence sample against?
2. We ask, could this suspect be a source of the DNA that we see in the ……..?
3. We undertake no statistical analysis in cases where the person is……. as a contributor to the evidence sample?
4. Converting the number of repeating base pair values we need to calculate their ….?
5. Two peaks on the EPG, represent?
6. The principle which underlies the product rule?

Answer 5

1. When comparing EPGs we compare the evidence sample against a reference sample.
2. We ask, could this suspect be a source of the DNA that we see in the evidence sample.
3. We undertake no statistical analysis in cases where the person is excluded as a contributor to the evidence sample?
4. Converting the number of repeating base pair values we need to calculate their population frequency
5. Two peaks on the EPG, represent a heterzygous locus.
6. The principle which underlies the product rule was made by HArdy Weinberg,

Question 6

Product rule for heterozygous loci

Answer 6

2 x P x Q

Question 7

P and Q

Answer 7

• P is the frequency of one of the alleles we find in this particular profile, at this locus.
• Q is the frequency of one of the alleles we find in this particular profile at this locus.

Question 8

Product rule for homozygous loci?

Answer 8

P²

Question 9

Allele frequency reference databases

Answer 9

• In the UK we undertook reference sampling from a wide range of the population.
• In the USA – the FBI took DNA samples from around 200 recruits in the 1980s. In both examples we generated DNA profiles from these reference materials.
• These DNA profiles have, in turn been used to give us information on how common or how rare many of these alleles are.

Question 10

Final step in the product rule calculation

Answer 10

Multiply all of the frequencies to obtain the profile frequency and present it as 1 in X.

Question 11

1. We use this equation in a case of a homozygous peak?
2. Hardy Weinberg equilibrium tests within loci and also?
3. The match probability in the UK (Using the profile ceiling principle) is 1 in
4. Multiplying allele frequencies together gives us?
5. In the profile frequency, what must we do with the locus frequencies?
6. Multiplying the locus frequencies together provides us with the?

Answer 11

1. We use p squared in a case of a homozygous peak.
2. Hardy Weinberg equilibrium tests within loci and also across loci.
3. The match probability in the UK (Using the profile ceiling principle) is 1 in 1,000,000,000.
4. Multiplying allele frequencies together gives us locus frequency.
5. In the profile frequency, we must multiply together the locus frequencies.
6. Multiplying the locus frequencies together provides us with the profile frequency.