Forensics

Chapter 8

Central Points

DNA testing can determine identity

DNA profiles are constructed in specialized laboratories

DNA profiles used in courts

Ancestry can be determined by testing DNA

May refuse to give sample to the police

Case A: DNA Frightens Victim

Five years ago, Margaret was attacked and beaten

ID William in a line up by his “eyes”

William claimed innocence but was convicted and sentenced to 25 years

New DNA testing using PCR • Blood on victim’s clothing not William’s

• He is asking to be released from prison

8.1 How Is DNA Tested?

In 1975, in England

Dr. Alec Jeffreys developed DNA fingerprinting to compare DNA profiles from different individuals

Used to solve murders (see Narborough Village Murders)

8.2 DNA Profile (DNA Fingerprint)

Variations in:• Minisatellites (repeated sequences, 10–100 base

pairs)

• Short tandem repeats (STRs) (2–9 base pairs)

• Single nucleotide polymorphisms (SNPs)

DNA variations can be used to create DNA profile and ID individuals

Used in criminal cases, paternity lawsuits, and studies of human evolution

Minisatellites

Short tandem repeats (STRs)

Single nucleotide polymorphisms (SNPs)

Variations in DNA

...CCTGACTTAGGATTGCCA...

Methods to Prepare DNA Profiles

Two common methods:

Restriction fragment length polymorphism (RFLP) analysis

Polymerase chain reaction (PCR)

RFLP Analysis

Large amount of blood or tissue

Steps:

1.DNA extracted

2.Cut into fragments using restriction enzymes

3.Fragments loaded on agarose gel

4.Electrical current passed through gel, DNA fragments migrate and separating by size

5.Pattern fragments photographed and compared to others

p. 132

4 The gel is immersed in a liquid, and an electric current is applied across the gel from top to bottom.

Stepped Art

3 The DNA is loaded onto a gel.

5 DNA fragments move through the gel and separate by size. Smaller fragments migrate furthest.

1 DNA is extracted from cells belonging to three different people.

2 Each sample is cut with a specific restriction enzyme.

6 The DNA bands are visualized on film and analyzed.

Steps in RFLP

RFLP

PCR

Small amount of DNA sample

Steps in one cycle:

1.DNA placed in solution

2.Heated, double-stranded DNA separates

3.Temperature lowered, primers mixed with DNA

4.Primers pair with complementary regions on DNA

5.DNA polymerase uses nucleotides to synthesize double-stranded DNA molecule

2 DNA is heated and separates into single strands.

p. 133

1 DNA is recovered for PCR testing. Primers

3 Primers are added and bind with complementary regions in the DNA.

4 Primers serve as starting points for new DNA.

5 New double stranded DNA is formed.

a. One PCR cycle Stepped Art

2 One cycle

p. 133

1 Original sample

3 Two cycles

b. Multiple PCR cycles

4 Three cycles

Stepped Art

Steps in PCR

Using PCR to Increase DNA

STR: Allele, a Unique Number of Repeats

2–9 nucleotides long

Example of three alleles:• Allele 1: …AGA…

• Allele 2: …AGAAGAAGA…

• Allele 3: …AGAAGAAGAAGAAGA…

Analysis of several STR alleles show frequency of specific combinations

PCR for DNA Profile

Steps:

1.DNA sample analyzed for STR alleles present in population members

2.Analyze population frequency, how often combinations of alleles present

3.Population frequencies for each STR allele multiplied to estimate probability

Combined Frequencies

DNA Samples (1)

DNA Samples (2)

Case A Questions

Should William get a new trial?

What are the benefits and challenges of using DNA profiles?

How reliable is DNA evidence?

Animation: Studying and Manipulating Genomes (DNA fingerprinting)

Animation: Whodunnit? A DNA fingerprint crime

8.3 DNA Profiling and U.S. Courts

Landmark case: Frye v. U.S.

DNA entered U.S. court system in 1989

Approved for use in courts all over the country

Evidence may be challenged

How Are DNA Profiles Used?

Material from crime scene analyzed, compared to other samples or data base

Combined DNA Index System (CODIS) panel of 13 STRs for DNA profiles

~30% of DNA profile results clear innocent people

DNA Profile Comparison

DNA Databases

Since 1998, FBI enters DNA profile from felons

More than 1,700,000 profiles

DNA databases becoming important tools in solving crimes

8.4 Other Uses for DNA Profiles

Czar Nicholas II and family

Remains of military personnel killed in action

Victims of 9/11 and Katrina

Paternity identification

Trace our ancestry

Czar Nicholas II

Tracing Ancestry

Mitochondrial DNA testing• Mitochondria organelle with DNA

• Maternal inheritance, passed from mother to children

• Use haplotypes to trace maternal ancestry

Y chromosome testing• Y chromosome passed father to son

• Use haplotypes to trace paternal inheritance

Genographic Project sponsored by National Geographic Society

Mitochondrial Inheritance

Y Chromosome Inheritance

Case B: Samples Asked of All

Two women murdered in a small community

Males > age 17 to provide sample for DNA dragnet

Murderer was identified

What are the important issues in this request?

8.5 Legal and Ethical Issues