Dr. Carolyn Hancock

Forensic DNA Profiling:

An introduction to scientific principles

Presentation to the Portfolio Committee of Police

Criminal Law (Forensic Procedures) Amendment Bill B9-2013

Crime Scenes: Types of evidence

BLOODWEAPONSFINGERPRINTS

CIGARETTE BUTT BOTTLECLOTHING

SHOE PRINTS SEMEN

Everyone has a unique DNA profile (except

identical twins)

DNA in every cell of an individual is

identical

An individual’s DNA profile remains

unchanged throughout life

Useful characteristics of DNA

Fighting crime with science

DNA in crime scene investigation

Identify or exclude a suspect

Identify missing persons

Because the DNA of all people is different we can:

Fighting crime with science

Body fluids used in DNA profiling

Each chromosome:contains one DNA molecule

Each chromosome:contains one DNA molecule

ChromosomesChromosomes

CellsCells

Each body cell:contains 46 chromosomes

Each body cell:contains 46 chromosomes

salivasaliva

bloodblood

semensemen

Body fluids containing DNABody fluids containing DNA

samplesample

samplesample

samplesample

46 chromosomes46 chromosomes

5% functional genes5% functional genes

95% non functional DNA95% non functional DNA

DNA

Used for DNA profiling

Non functional DNA:High proportion of sequence variationEnough differences to make DNA of each person unique

Non functional DNA:High proportion of sequence variationEnough differences to make DNA of each person unique

AA TTGGCCAA CC CC AA

4 different molecules for building blocks (A, C, G and T)4 different molecules for building blocks (A, C, G and T)

Sequences are specific for functional genesToo little variation to be useful for DNA profiling

Sequences are specific for functional genesToo little variation to be useful for DNA profiling

TT TT GG GG GG TTAACC

DNA is double stranded DNA is double stranded

DNA profiling: How it works

Extract DNA out of sample and amplify DNA segments

Separate and visualize DNA fragments in conducting gel

Store profile as Numbers 5/7

Longer molecule – 7 repeatsLonger molecule – 7 repeats

Shorter molecule – 5 repeatsShorter molecule – 5 repeats

DigitalProfile

DigitalProfile

ATCTATCT ATCTATCT ATCTATCT ATCTATCT ATCTATCT ATCTATCT ATCTATCT

ATCTATCT ATCTATCT ATCTATCT ATCTATCT ATCTATCT

Chr 11

Chr 11

Region (locus) on Chromosome 11 with repeated segments – referred to as microsatellite - STR Region (locus) on Chromosome 11 with repeated segments – referred to as microsatellite - STR

Machine Profile

Machine Profile

Suspect 1

Suspect 2

Evidence

Possible outcomes of a DNA result

Exclusion – no match

Inclusion Match

Male: 13-13; 14-15; 16-12; 13-10; 13-15; 16-16Profile entered onto relevant index on DNA Database

STR Typing at laboratory

Reference Profile Analysis

REFERENCE SAMPLE DESTRUCTION

Sample Collection

Buccal swab

Information stored on DNA Database

• A DNA profile is stored on the Database as a

list of letters and numbers

These numbers are not associated with any

physical trait, genetic disease or human behavioral

pattern

• Only personal information: sex

• Case number, sample bar code, laboratory

number

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