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7BBFM128: Forensic Biology > Relationship Testing > Flashcards

Relationship Testing Flashcards

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1
Q

What can relationship testing also be known as?

A

Kinship analysis

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2
Q

What is relationship testing used for?

A
  • Used to assess whether (and how) people may be genetically related to one another
  • Relatedness calculations are carried out based on number of alleles shared
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3
Q

What are some applications for relationship testing?

A

o Paternity testing
o Immigration cases
o Missing person identification (disaster victim identification)
o Inheritance disputes

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4
Q

In relationship testing, what is it that is being investigated?

A

Genetic relatedness

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5
Q

What evidence are we searching for with relationship testing?

A
  • We are looking for evidence to suggest that one individual is direction descended from the other
    o Children are separated from their parents by **one generation **
    o Grandchildren and separated from their grandparents by **two generations **
  • Or that individuals share a very **recent common ancestor **
    o Full siblings share two parents
    o Half-siblings share one parent
    o First cousins share two grandparents
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6
Q

Whats recombination?

A
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7
Q

How was relationship testing first performed?

A
  • First performed **using blood groups **
    o Very useful for excluding a man as the potential father as mutations do not occur
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8
Q

Why are mutations more common in STR than in blood groups?

A

STR in bit of DNA non-coding and blood groups are proteins, there for a purpose, for survival and function

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9
Q

What was the first application of ‘DNA fingerprinting’?

A

**First application of ‘DNA fingerprinting’ (RFLP testing) was an immigration case (maternity) **
o A boy born in Britain to a Ghanian mother had just returned to the UK after living with his father in Ghana for a short while
o About 20 blood groups- one exclusion (Duffy)
o Mother Christiana phenotype Fy(a+)
o Child Andrew phenotype Fy(b+)

  • Certainly, closely related, but was Christiana his aunt?
    o Bands shared on average 26% of the time
  • Mother v aunt more than one billion times more likely
  • Currently routinely carried out using STR typing
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10
Q

In what situations would you want to determine paternity (maternity)?
Give subgroup examples for each

A
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11
Q

What are the non-paternity rates?

A
  • Quoted rates vary 1%-30%
    o Methodology often not validated
    o Prevalent in testing laboratories- 15%
     Paternity questioned
    o Recent studies
     **1-3% estimated in the UK **
  • Large variation seen between countries, cultures and different social statis
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12
Q

How is the assessment of paternity done in the following case?

  • Mother names a man as the father of her child
  • If the ‘mother’ is the true mother, then mother and child must have an allele in common
  • Tested man is not excluded from paternity
A
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13
Q

What is the equation for likelihood ratio?

A
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14
Q

What is the chance of the tested man being the father?

A
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15
Q

How would you work out the probability of independent events?

A
  • When both A and B occur together
    A n B= p(A) x p(B)
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16
Q

What do indepent events allow in forensic genetic situations?

What are some limitations to this?

A
  • Allows likelihood ratios from independent genetic systems to be combined into a total LR by multiplication
    o 1.89 x 1.45 x ….. = 10,0000 (minimum)
    o The **‘combined paternity index (CPI)’ **

*** Caveats **
o Same chromosome- genes may be linked (VWA and D12S391 are linked)
o Different chromosomes- non-random assortment due to population structure

17
Q

Tell me about the paternity index (likelihood of paternity)

A

**Paternity index (likelihood of paternity)= 10,000 **
* The tested man is 10,000 times more likely to be the father than an unrelated man (no)

  • The types observed are10,000 time more likely to be seen in a trio of true father, mother, and child than in a trio of an unrelated man, mother and child
  • A laboratory should define a minimum PI- 100 standard in US- most labs look for at least 10,000
18
Q

What does the report day regarding PI?

A

**Some paternity reports may say: **
o Paternity index (LR)= 10,000
 LR= likelihood ratio

o Probability of paternity (W)= 99.99%
 W (Wahrscheinlichkeit)- probability

o Implies that these are two ways of expressing the same thing, which they are NOT

19
Q

Given the evidence, what are the offs that the man we have tested IS the father?
How is this caluclated and what is the equation for this?

A
20
Q

Tell me what Prior odds and what they depend on

A
21
Q

A range of priors?

A
  • Assuming that the CPI is 10,000
  • ‘Assuming a prior of between 10% and 90% the paternity index is somewhere between 1,111 and 900,000’
    o 10% prior= 0.1/0.9 x 10,000 = 1,111
    o 90% prior= 0.9/0.1 x 10,000= 90,000
  • A pragmatic approach
    o We say- 10,000, ‘assuming no other relevant evidence’
22
Q

But is that 99.99%?

A
  • Probability of paternity (W)
    = **(Pi LR) / (Pi LR + 1) – (Essen-Möller equation) **

Where Pi is the prior odds
Assuming a 50% prior
o **Probability (W) = LR / LR+1 **
o If paternity index = 10,000
 Probability of paternity= 10,000 / 10,001
= 0.9999
= 99.99%
o Can never reach 100% and always include non-genetic evidence- unlike the PI

23
Q

What are the pros/cons of paternity of a diagnositc test?

A
  • A good diagnostic test is one which will minimise the number of false positive and false negative results
  • **False positive- **telling someone that they have a disease, when in fact they do not
    o Significant false positive rates for mammograms
    * False positive- telling someone that they are the father of a child when they are not
    o Not tolerable- an easy mistake in some circumstances

* False negatives- telling someone that they do not have the disease, when in fact they do
o Significant false negative rates for cervical cytology
o Significant false negative rates for COVID19
* False negatives- telling someone that they are not the father of a child, when they are
o Not tolerable- but an easy mistake
* Unlike diagnostic tests- the ‘one off’

24
Q

False positive paternity

A
  • Man, mother, and child tested
  • CPI= 21,000
  • Probability of paternity= 99.995%
  • PI is the genetic odds in favour of paternity
  • Man is the brother of the true father
  • “Offers very strong evidence paternity providing a close relative of the tested man is not a possible father”
25
Q

Example case study

A

*** Disputed paternity **
o CPI 87,000
o Probability of paternity 99.998%

* What were the prior odds?
o Intercourse acknowledged with mother
o Fertility undisputed- 4 children
o Vasectomy 5 years previously
o Regular un-protected sex with wife

**Vasectomy **
*** Failure rate **
o Literature report 6 in 14,000 (1 in 2400) 0.04%
 Failure defined as sperm seen
o Literature report (1 in 85,000) 0.00118%
 Failure defined as child born to vasectomised man (6 men)

Very low prior odds
* 0.0000118 (very low prior odds) x PI 87,000 (Very strong evidence towards paternity) = 1.0266 (? Paternity)
* Judgement
**o He is the father **

26
Q

Whats the first rule of inheritance leading to exclusion of paternity?

A
  • When something is found in a child that is not in the child’s mother it must come from the father
  • If the tested man does not have it then he normally cannot be the father
  • Why ‘normally’
    o We can’t test the sperm that fertilised the egg
27
Q

Why do I need to be tested? I am his mother?

A
  • Tested man is VWA (16,19), child is VWA (13,16)
    o Tested man is not excluded from paternity
  • Tested man is VWA (16,19), child is VWA (13,16) and mother is VWA (16,18)
    o Tested man is very probably excluded from paternity
28
Q

Is this man excluded?

A
29
Q

Including mutations in paternity

A
30
Q

What is the second rule of inheritance?

A

Second rule of inheritance leading to exclusion of paternity (does not involve the mother)
* When something is found in the alleged father that should be inherited by his children, if a child does not have it he may not be the father

31
Q

Is this man excluded?

A
32
Q

What is the estimated incidence of ‘silent’ alleles?

A
33
Q

How would you deal with silent alleles in paternity?
What could they be due to?

A

*** Could be a primer mutation **
o Try to repeat system using different primers
o Check the height of the allele peaks in comparison with others in the profile

*** Could be a deletion of part of the chromosome **
o Very rare (other than the Y chromosome)

34
Q

Reporting non-paternity

A

Laboratory needs to determine rules
o Greater than 2 or 3 exclusions
o Number will depend on chance of a mutation- more tests more chances

35
Q

Tested man is excluded from paternity, UNLESS what?

A

*** Mutation **

  • Man is chimera (an organism containing a mixture of genetically different tissues, formed by processes such as fusion of early embryos, grafting or mutations ie., sperm may be different to blood)
  • **Wrong sample **
    o Wrong man presented (identity error)
    o Sample wrongly labelled (sampler error)
    o Wrong sample tested (laboratory error)
  • Wrong results
    o Laboratory error (interpretation, technical)
    o Laboratory error (clerical)
36
Q

Is this man excluded?

A
37
Q

Is this man excluded?

A
38
Q

Special cases

A
  • Deficient family- deceased parent
    o 2-person test
  • Deficient family- deceased parent
    o Use a sibling of deceased- * Deficient family- deceased parent
    o 2-person test
  • Deficient family- deceased parent
    o Use a sibling of deceased- avuncular index (AI)- An avuncular index is calculated using these genetic profiles. If the avuncular DNA index is less than 1.00 it indicates that the tested persons are not related
    o = (locus PI + 1)/2
  • Immigration case- is a close relative the father?
    o Evaluate the ratio combined PI/ combined AI
  • Deficient family- deceased parent
    o Use parent(s) of deceased
     Substitute each grandparent as putative father and take the mean of the two PIs for each test
     If only one grandparent, use PI=1 for the other grandparent
  • Y chromosome STRs and MtDNA
    o Must use haplotype frequencies
    o Can be combined with autosomal data
  • Need to caution against the chance match probabilities because of population substructure
  • ISFG recommendations on biostatistics in paternity testing (2007) FSI genetics 1: 223- 231
39
Q

Familias- a programme for relationships

A
  • Choose a population frequency
  • Check database choice
  • Import your data
  • Import family
  • Import data
  • Set up hypothesis
  • Hypothesis 1 (tested man is the father)
  • Hypothesis 2 (an unrelated man is the father)
  • Result
    o Look at LR, anything over 1x109 is billion
    o So this would mean the change that, that male is the father is over 1 billion

7BBFM128: Forensic Biology (19 decks)

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