Genetic Statistic Application in Forensic Science

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Genetic Statistic Application in Forensic Science
Arthur J. Eisenberg, PhD Professor and Chairman
Department of Forensic and Investigative
Genetics Co-Director UNT Center for Human
Identification Institute of Investigative
Genetics University of North Texas Health Science
Center Fort Worth, Texas USA
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The Science of DNA for Human Identification
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DNA is Responsible for Transmitting Hereditary
Characteristics
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Human Cells Contain Two Kinds of DNA
Nuclear DNA and Mitochondrial DNA


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The Nucleus Contains 23 Pairs of Chromosomes

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DNA in the Cell
Target Region for PCR
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(No Transcript)
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DNA Amplification with the Polymerase Chain
Reaction (PCR)
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PCR Copies DNA Exponentially through Multiple
Thermal Cycles
Original DNA target region
In 32 cycles at 100 efficiency, 1.07 billion
copies of targeted DNA region are created
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13 CODIS Core STR Loci
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Alleles Contain Varying Numbers of 4 Base Pair
Repeats
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Typical DNA Profile
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Multiplex PCR

• Over 15 Markers Can Be Copied at Once
• Sensitivities to levels less than 1 ng of DNA
• Ability to Handle Mixtures and Degraded Samples
• Different Fluorescent Dyes Used to Distinguish
  STR Alleles with Overlapping Size Ranges

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310 Genetic AnalyzerCapillary Electrophoresis
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Current CODIS Acceptable Kits
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AmpFlSTR Identifiler
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Current CODIS Acceptable Kits
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COmbined DNA Index System
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CODIS Mission

• The FBI Laboratory's Combined DNA Index System
  (CODIS) blends forensic science and computer
  technology into an effective tool for providing
  investigative leads to assist in solving crimes.
• CODIS enables federal, state, and local crime
  labs to exchange and compare DNA profiles
  electronically, thereby linking crimes to each
  other and to convicted offenders, as well as in
  the identification of missing persons and human
  remains.

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What is a Database?

• A database is an organized file or files of data
  that can be searched to retrieve information
• DNA databases compare crime scene evidence to a
  database of DNA profiles obtained from known
  individuals to provide the Police with
  investigative leads

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CODIS Architecture
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CODIS INDICES

• OFFENDER
• Convicted Offenders
• Arrestee
• FORENSIC
• Forensic Crime Scene Samples
• MISSING PERSONS
• Unidentified Human Remains
• Missing Persons Direct Reference Samples
  (baby teeth, tooth brush, hair brush, etc.)
• Family Reference Samples

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Question 1 How common or rare is the evidence
profile?

• This can be calculated by either the random match
  probability (RMP) or by using the likelihood
  ratio (LR) using the Hardy-Weinberg Equilibrium
  Formula.

For homozygous loci, a Theta correction (?) is
used to account for population substructure
Homozygous locus frequency p2p(1-p)?
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PART 2
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Paternity Exclusion
Obligate allele
Obligate allele
Dual Obligate alleles
X
X
X
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Paternity Exclusion
Obligate allele
Obligate allele
Obligate allele
Obligate allele
X
X
X
X
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Paternity Exclusion
Dual Obligate alleles
Obligate allele
X
X
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Paternity Inclusion
Obligate allele
Obligate allele
Obligate allele
Obligate allele
Obligate allele
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Paternity Inclusion
Obligate allele
Obligate allele
Obligate allele
Obligate allele
Dual Obligate alleles
Dual Obligate alleles
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Paternity Inclusion
Obligate allele
Obligate allele
Obligate allele
Obligate allele
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If the Alleged Father Cannot be Excluded
Several Statistical Values are Calculated to
Assess the Strength of the Genetic Evidence
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Paternity Index

• Summarizes the genetic information provided by
  the DNA analysis of the Mother, the Child, and
  the Alleged Father
• The Numerator is the Probability of observing the
  genetic results for the three individuals tested
  under the assumption that they are a true trio
• The Denominator is the probability of observing
  the same genetic results for the three
  individuals under the assumption that they are a
  false trio.

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Probability of Paternity

• The probability of paternity is a measure of the
  strengths of ones belief in the hypothesis that
  the tested man is the father.
• The correct probability must be based on all of
  the evidence in the case.
• The non-genetic evidence comes from the testimony
  of the mother, tested man, and other witnesses.
• The genetic evidence comes from the DNA paternity
  test.

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Probability of Paternity

• The prior probability of paternity is the
  strength of ones belief that the tested man is
  the father based only on the non-genetic
  evidence.
• P Prior Probability it is a number greater
  than 0 and less than or equal to 1. In many
  criminal proceedings the Probability of Paternity
  is not admissible.
• In criminal cases, the accused is presumed
  innocent until proven guilty. Therefore, the
  defense would argue that the Prior Probability
  should be 0. You cannot calculate a posterior
  Probability of Paternity with a Prior Probability
  of 0.

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Probability of Paternity

• In the United States, the civil court system has
  made the assumption that the prior probability is
  equal to 0.5.
• The argument that is presented is that the tested
  man is either the true father or he is not. In
  the absence of any knowledge about which was the
  case, it is reasonable to give these two
  possibilities equal prior probabilities.

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DNA Paternity Testing

• No test available can prove with a probability of
  paternity or maternity of 100 that a man or
  woman is the biological parent of a child.
• Currently available DNA testing will routinely
  provide greater than a 99.9999 probability of
  paternity/maternity when the biological mother is
  tested in conjunction with the child and the
  alleged father. This is in excess of the 99 or
  99.9 requirement of most U.S. civil courts.

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Probability of Exclusion

• The probability of exclusion (PE) is defined as
  the probability of excluding a random individual
  from the population given the alleles of the
  child and the mother.
• The genetic information of the tested man is not
  considered in the determination of the
  probability of exclusion
• The probability of exclusion (PE) is equal to the
  frequency of all men in the population who do not
  contain an allele that matches the obligate
  paternal allele of the child.

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PART 3
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Arthur J. Eisenberg, PhD Professor and Chairman
Dept of Forensic and Investigative
Genetics Co-Director UNT Center for Human
Identification Institute of Investigative
Genetics University of North Texas Health Science
Center Fort Worth, Texas USA 817
735-0555 Arthur.eisenberg_at_unthsc.edu