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DNA > Topic 7 - Mixtures > Flashcards

Topic 7 - Mixtures Flashcards

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1
Q

what is meant by a motif

A

the repeating unit at STR markers - typically consisting of a 3, 4 or 5 base repeat

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2
Q

when was DNA 17 introduced and what did it replace

A

2014

replacing SGM+

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2
Q

how can we identify a mixed DNA profile

A

when there is three or more peaks per locus

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3
Q

what is meant by a DNA mixture

A

a sample containing DNA from two or more individuals

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4
Q

what represents one of the greatest challenges in Forensic DNA analysis

what does this reduce

A

interpreting DNA mixtures

reducing reliability of evidence

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5
Q

when are the DNA profiles better quality/have higher peak height

A

when there is more available DNA

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6
Q

what are three problems with DNA mixtures

A
  1. they dont tell you how or what order the DNA was deposited
  2. if no statistical evaluation of the profile can be done the evidence can be considered inconclusive
  3. there is then the use of probabilistic genotyping to be able to apply the LR
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7
Q

in crime scenes are we more likely to obtain single source or mixed DNA profiles

A

mixed profiles

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7
Q

why have DNA mixtures and trace DNA become prevalent in casework

but what issue does this also lead to

A

techniques are so sensitive we can generate a profile from just a few skin cells

DNA is likely to be present in small amounts from many people as we shed DNA when doing simple things like talking or touching

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8
Q

what is the probability of A and B when the two events aren’t independent

when they are independent what do we use

A

probability of A x probability of B given A

the product rule (in single source profiles)

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8
Q

is random match probability the statistical approach for mixed DNA profiles

what is

A

no

the likelihood ratio
- find probability of prosecution and defence hypothesis to evaluate the weight of the evidence

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9
Q

what chromosome is the locus D8S1179 found on

A

chromosome 8 - the first number

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10
Q

what is the first thing to do when analysing DNA evidence

A

set your two propositions

prosecution and defence

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11
Q

how do we covert a probability to the odds

A

odds = probability/(1-probability)

e.g 0.6/0.4 = 6/4 the odds are 6 to 4

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11
Q

when look at a DNA profile of two sources what do we try and label the two contributors as

how would you tell these apart

A

the major and the minor mixture

the major is likely to have higher peaks on the EPG as the relative ‘amount’ of DNA is greater

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11
Q

what is meant by a prejudicial view of DNA evidence

A

presenting a number of matching components between a defendant and the DNA mixture

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12
Q

all forensics is what dependent

A

CONTEXT

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13
Q

In a two person mixture profile, when is the profile difficult to interpret

A

when there are only trace amounts of DNA as it is hard to know if the peak represents one or more individuals

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14
Q

what are the three main questions to ask yourself when analysing DNA mixtures

A
  1. how many people contributed to the profile
  2. how much DNA did each contributor contribute
  3. how degraded is the DNA, if at all
15
Q

what should you do if a DNA profile is too complex to interpret at all

A

state this in your report and say why you can’t interpret the profile reliably

16
Q

what are the two uncertainties when interpreting DNA mixtures

A
  1. when is a peak a significant peak - due to trace amount of DNA, alleles can drop out
  2. which peak belong to which contributor - just because one individual alleles are all in a mixture does not necessarily mean they contributed as we can’t tell which allele belongs to which contributor
17
Q

if a scientist is unaware of the proposition being put forward by the defence what do they use in the LR

A

a proxy proposition

17
Q

what are the three steps in analysing single source DNA profile s

A

eliminate
determine a match
calculate random match probability and then LR

18
Q

how do we calculate LR from the Random match probability in single source profiles

A

1/RMP

19
Q

what two types of samples are likely to see drop in/our

A

low amounts/trace
degraded/poor quality sample

20
Q

what do AT and ST stand for in DNA analysis

A

AT = analytical threshold = limit of detection on the equipment = distinguish between an allele peak or artefact

ST = stochastic threshold = if above this is is suggested to be an allele peak and not a stochastic effect. if one peak above this then likely to be a homozygous peak but it between this and AT likely to be heterozygous and the other allele didn’t pick up or

21
Q

name two stochastic effects that can complicate mixture interpretation even further

in what two situations are these normally seen

A

drop in or drop out

often seen with low level or degraded DNA

21
Q

what does PGS stand for

what 3 things does this account for in mixture interpretation

A

probabilistic genotyping software

accounts for drop in and out, other stochastic effects and the fact that some alleles are more popular in a population than others

21
Q

what is probabilistic genotyping

A

a method used when we have a mixture but interpretation if difficult due to low levels of DNA or degradation

it is a biological modelling statistical theory that uses computer algorithms and probability distributions to infer genotypes for DNA profiles in forensic samples so we can calculate LR’s

22
Q

is the continuous or semi-continuous probabilistic genotyping method quicker

A

semi - continuous

22
Q

what does a probabilistic genotyping software produce

A

a likelihood ratio = an estimate of how likely it is to see this mixture if the suspect did contribute or didn’t

23
Q

what is the problem with probabilistic genotyping software

A

different labs use different software’s so they might end up seeing different results for the LR value

24
Q

what are the two types of probabilistic genotyping method (briefly explain each)

A

continuous = uses all data (allele an peak height info) and also biological parameters e.g peak height ratio, mixture ratio and stutter percentages to calculate probability of the observed peak heights

semi-continuous = same as above but doesn’t use biological parameters, accounts for allele drop in and out though

25
Q

what are the 7 Clayton Rules used in DNA mixture interpretation

A
  1. identify presence of mixture
  2. identify artefacts vs alleles
  3. identify number of contributors
  4. determine approx ratio of components in the mixture
  5. determine possible genotype combos (which two alleles go together) for the different contributors
  6. compare possible genotypes to reference samples
  7. perform statistical analysis
26
Q

what is said to be a good balance peak height ratio

A

0.6

27
Q

what are two types of artefact that may be seen in DNA profiles

A

pull up = from a dye bleeding into another channel

stutter

27
Q

what can probabilistic genotyping tell you (1)

and what can’t it (2)

A

who might have contributed to a mixture

how or when the DNA got there

27
Q

what are two things to use to help suggest how many contributors are there to a mixture

A

PHR’s = PEAK HEIGHT RATIOS

or

the number of alleles at a locus

28
Q

what type of method is probabilistic genotyping

why does this restrict the DNA interpretation

A

a binary method

unable to deal with complex, low level mixed profiles

29
Q

what does the ability and reliability to interpret a mixture rely on

A

the context of the case

these samples are often challenging but can still give powerful evidence

30
Q

what is used to suggest the number of contributors to a mixture

if we see a locus with
a) 3 or more alleles
b) more than 4 alleles
what can we say about the number of contributors

A

the maximum number of alleles at one locus

a) more than one individual
b) three or more individuals

31
Q

what is meant by allele dropout

what does this result in

A

when a peak in the EPG falls below the analytical threshold of the instrument

AT is normally set to a small amount of LOD of instrument

results in an incomplete profile being obtained

32
Q

the more the people that appear to be in the mixture the less…….

A

you can be sure about the number of contributors

33
Q

is the POI (person of interest) is excluded from being a contributor what is then not relevant

A

a statistical evaluation

34
Q

what does the profile frequency represent

A

how common is the profile in the population

DNA (16 decks)

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