Topic 7 - Mixtures Flashcards
what is meant by a motif
the repeating unit at STR markers - typically consisting of a 3, 4 or 5 base repeat
when was DNA 17 introduced and what did it replace
2014
replacing SGM+
how can we identify a mixed DNA profile
when there is three or more peaks per locus
what is meant by a DNA mixture
a sample containing DNA from two or more individuals
what represents one of the greatest challenges in Forensic DNA analysis
what does this reduce
interpreting DNA mixtures
reducing reliability of evidence
when are the DNA profiles better quality/have higher peak height
when there is more available DNA
what are three problems with DNA mixtures
- they dont tell you how or what order the DNA was deposited
- if no statistical evaluation of the profile can be done the evidence can be considered inconclusive
- there is then the use of probabilistic genotyping to be able to apply the LR
in crime scenes are we more likely to obtain single source or mixed DNA profiles
mixed profiles
why have DNA mixtures and trace DNA become prevalent in casework
but what issue does this also lead to
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
what is the probability of A and B when the two events aren’t independent
when they are independent what do we use
probability of A x probability of B given A
the product rule (in single source profiles)
is random match probability the statistical approach for mixed DNA profiles
what is
no
the likelihood ratio
- find probability of prosecution and defence hypothesis to evaluate the weight of the evidence
what chromosome is the locus D8S1179 found on
chromosome 8 - the first number
what is the first thing to do when analysing DNA evidence
set your two propositions
prosecution and defence
how do we covert a probability to the odds
odds = probability/(1-probability)
e.g 0.6/0.4 = 6/4 the odds are 6 to 4
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
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
what is meant by a prejudicial view of DNA evidence
presenting a number of matching components between a defendant and the DNA mixture
all forensics is what dependent
CONTEXT
In a two person mixture profile, when is the profile difficult to interpret
when there are only trace amounts of DNA as it is hard to know if the peak represents one or more individuals
what are the three main questions to ask yourself when analysing DNA mixtures
- how many people contributed to the profile
- how much DNA did each contributor contribute
- how degraded is the DNA, if at all
what should you do if a DNA profile is too complex to interpret at all
state this in your report and say why you can’t interpret the profile reliably
what are the two uncertainties when interpreting DNA mixtures
- when is a peak a significant peak - due to trace amount of DNA, alleles can drop out
- 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
if a scientist is unaware of the proposition being put forward by the defence what do they use in the LR
a proxy proposition
what are the three steps in analysing single source DNA profile s
eliminate
determine a match
calculate random match probability and then LR
how do we calculate LR from the Random match probability in single source profiles
1/RMP
what two types of samples are likely to see drop in/our
low amounts/trace
degraded/poor quality sample
what do AT and ST stand for in DNA analysis
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
name two stochastic effects that can complicate mixture interpretation even further
in what two situations are these normally seen
drop in or drop out
often seen with low level or degraded DNA
what does PGS stand for
what 3 things does this account for in mixture interpretation
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
what is probabilistic genotyping
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
is the continuous or semi-continuous probabilistic genotyping method quicker
semi - continuous
what does a probabilistic genotyping software produce
a likelihood ratio = an estimate of how likely it is to see this mixture if the suspect did contribute or didn’t
what is the problem with probabilistic genotyping software
different labs use different software’s so they might end up seeing different results for the LR value
what are the two types of probabilistic genotyping method (briefly explain each)
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
what are the 7 Clayton Rules used in DNA mixture interpretation
- identify presence of mixture
- identify artefacts vs alleles
- identify number of contributors
- determine approx ratio of components in the mixture
- determine possible genotype combos (which two alleles go together) for the different contributors
- compare possible genotypes to reference samples
- perform statistical analysis
what is said to be a good balance peak height ratio
0.6
what are two types of artefact that may be seen in DNA profiles
pull up = from a dye bleeding into another channel
stutter
what can probabilistic genotyping tell you (1)
and what can’t it (2)
who might have contributed to a mixture
how or when the DNA got there
what are two things to use to help suggest how many contributors are there to a mixture
PHR’s = PEAK HEIGHT RATIOS
or
the number of alleles at a locus
what type of method is probabilistic genotyping
why does this restrict the DNA interpretation
a binary method
unable to deal with complex, low level mixed profiles
what does the ability and reliability to interpret a mixture rely on
the context of the case
these samples are often challenging but can still give powerful evidence
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
the maximum number of alleles at one locus
a) more than one individual
b) three or more individuals
what is meant by allele dropout
what does this result in
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
the more the people that appear to be in the mixture the less…….
you can be sure about the number of contributors
is the POI (person of interest) is excluded from being a contributor what is then not relevant
a statistical evaluation
what does the profile frequency represent
how common is the profile in the population
