Genetic Statistic Application in Forensic Science

Arthur J. Eisenberg, PhDProfessor and Chairman Department of Forensic and Investigative Genetics

Co-Director UNT Center for Human IdentificationInstitute of Investigative Genetics

University of North Texas Health Science CenterFort Worth, Texas USA

The Science of DNA for Human Identification

DNA is Responsible for Transmitting Hereditary Characteristics

Human Cells Contain Two Kinds of DNA

*

*

Nuclear DNA and Mitochondrial DNANuclear DNA and Mitochondrial DNA

The Nucleus Contains 23 Pairs of Chromosomes

DNA in the Cell

Target Region for PCRTarget Region for PCR

Make copies (extend primers)

Starting DNA

Template

5’

5’

3’

3’

5’

5’

3’

3’

Add primers (anneal) 5’3’

3’5’

Forward primer

Reverse primer

Separate strands

(denature)

5’

5’3’

3’

DNA Amplification with the Polymerase Chain Reaction (PCR)

In 32 cycles at 100% efficiency, 1.07 billion copies of targeted DNA region are created

In 32 cycles at 100% efficiency, 1.07 billion copies of targeted DNA region are created

PCR Copies DNA Exponentially through Multiple Thermal Cycles

Original DNA target region

Thermal cycle

Thermal cycle

Thermal cycle

13 CODIS Core STR Loci

CSF1PO

D5S818

D21S11

TH01

TPOX

D13S317

D7S820

D16S539D18S51

D8S1179

D3S1358

FGAVWA

AMEL

AMEL

Alleles – Contain Varying Numbers of 4 Base Pair Repeats

Typical DNA Profile

D3S1358D3S1358 14,1614,16 D13S317D13S317 11,1211,12

vWAvWA 14,1914,19 D7S820D7S820 10,1010,10

FGAFGA 21, 2121, 21 D16S539D16S539 12,1312,13

D8S1179D8S1179 11,1511,15 THO1THO1 7, 97, 9

D21S11D21S11 30, 3230, 32 TPOXTPOX 8,128,12

D18S51D18S51 15,1715,17 CSF1POCSF1PO 11,1311,13

D5S818D5S818 12,1212,12 AmelogininAmeloginin X,YX,Y

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

310 Genetic AnalyzerCapillary Electrophoresis

Current CODIS Acceptable KitsCurrent CODIS Acceptable Kits

D8S1179 D21S11 D7S820 CSF1PO

D3S1358TH01

D13S317 D16S539 D2S1338

D19S433 D18S51TPOX

VWA

AMEL D5S818 FGA

GS500 LIZ size standard

6FAM (blue)

VIC (green)

NED (yellow)

PET (red)

LIZ (orange)

AmpFlSTR® Identifiler™

Current CODIS Acceptable KitsCurrent CODIS Acceptable Kits

COMBINED DNA INDEX SYSTEM

• 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.

CODIS MISSION

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

CODIS ARCHITECTURE

National

LDISTampa, FL

LDISTallahassee, FL

FDLECalDOJ

LDISOrlando, FL

DPS

NDIS

SDIS

LDIS

California Florida Texas

OFFENDERConvicted OffendersArrestee

FORENSICForensic Crime Scene Samples

MISSING PERSONS• Unidentified Human Remains• Missing Persons Direct Reference Samples

(baby teeth, tooth brush, hair brush, etc.)• Family Reference Samples

CODIS INDICES

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.

freq(A1) = p1 freq(A2) = p2

A1A1 = p12 A1A2 = 2p1p2 A2A2 = p2

2

p12 + 2p1p2 + p2

2 = 1

Homozygous locus frequency = p2+p(1-p)

For homozygous loci, a Theta correction () is used to account for population substructure

PART 2

X X X

Obligate allele Obligate alleleDual

Obligate alleles

Paternity Exclusion

X X X X

Obligate alleleObligate alleleObligate alleleObligate allele

Paternity Exclusion

X X

Paternity Exclusion

Obligate alleleDual

Obligate alleles

Paternity Inclusion

Obligate allele Obligate allele Obligate allele Obligate allele Obligate allele

Paternity Inclusion

Obligate allele Obligate allele Obligate allele Obligate alleleDual

Obligate allelesDual

Obligate alleles

Paternity Inclusion

Obligate allele Obligate alleleObligate allele Obligate allele

Several Statistical Values are Calculated to Assess the Strength of the Genetic Evidence

If the Alleged Father Cannot be Excluded

Paternity Index

Combined Paternity Index

Probability of Paternity

Probability of Exclusion

PI

CPI

W

PE

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.

Probability of Paternity• The probability of paternity is a measure of

the strengths of one’s 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.

Probability of Paternity• The prior probability of paternity is the strength of

one’s 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.

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.

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.

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.

PART 3

Arthur J. Eisenberg, PhDProfessor and Chairman

Dept of Forensic and Investigative GeneticsCo-Director UNT Center for Human Identification

Institute of Investigative GeneticsUniversity of North Texas Health Science Center

Fort Worth, Texas USA817 735-0555

Arthur.eisenberg@unthsc.edu