DNA Fingerprinting & Forensic Analysis

DNA Fingerprint & Genetic Profiling❖DNA fragments show unique patterns from one

person to the next.

❖A technique used by scientists to distinguish between individuals of the same species using only samples of their DNA.

DNA Fingerprint & Genetic Profiling❖The process of DNA fingerprinting was invented by

Alec Jeffreys at the University of Leicester in 1985.

❖Used in forensic science. ❖Use of scientific knowledge in legal situations.

Uses of DNA Profiling

◆ DNA profiling is used to solve crimes and medical problems

How is DNA used as evidence?• Each person’s DNA is different from other people (except identical twins).

• DNA collected from a crime scene can either link a suspect to the evidence or eliminate a suspect, similar to the use of fingerprints.

Sources of Biological Evidence

•Blood •Semen •Saliva •Urine •Hair •Teeth •Bone •Tissue

How is DNA used as evidence?• Each person’s DNA is different from other people (except identical twins).

• DNA collected from a crime scene can either link a suspect to the evidence or eliminate a suspect, similar to the use of fingerprints.

• DNA can identify a victim through DNA from relatives, even when no body can be found.

• DNA can link crime scenes together by linking the same perpetrator to different scenes locally, statewide, and across the nation.

• DNA can place an individual at a crime scene, in a home, or in a room where the suspect claimed not to have been.

• DNA can refute a claim of self-defense and put a weapon in the suspect's hand.

• Looking for matches between evidence, victim, and suspect.

•Must compare DNA profiles

DNA Profiles

◆ The DNA profile of each individual is highly specific.

◆ The chances of two people having exactly the same DNA profile is 30,000 million to 1 (except for identical twins).

RFLP

❖Restriction Fragment Length Polymorphism (RFLP) • Nucleotide sequence variations in a region of DNA that

generates fragment length differences according to the presence or absence of restriction enzyme recognition sites.

RFLP

❖The RFLP fragments can be separated by gel electrophoresis.

….there is a chance of 1 in 10 that this fragment occurs in many individuals…

Suppose that…………

…and.there is a chance of 1 in 20 that this fragment occurs in many individuals…

…and.there is a chance of 1 in 10 that this fragment occurs in many individuals…

…and.there is a chance of 1 in 30 that this fragment occurs in many individuals, but…

Chances of a match

…the probability of all 4 bands matching in any person other than the suspect is

1 in 10 x 1 in 20 x 1 in 10 x 1 in 30

= 1 in 10 x 20 x 10 x 30 That is 1 in 60,000

When a larger number of bands is involved, the probability that the suspect is not guilty becomes one in many thousands

- The chances of two people having exactly the same DNA profile is 3 trillion to 1 (except for identical twins).

Probability of a match

DNA Profiling can solve crimes◆ The pattern of the DNA profile is then compared

with those of the victim and the suspect.

◆ If the profile matches the suspect it provides strong evidence that the suspect was present at the crime scene ◆ It does not prove they committed the

crime- only shows suspect was present at the scene of the crime.

◆ If the profile doesn’t match the suspect then that suspect may be eliminated from the crime.

Example

◆ A violent murder occurred. ◆ The forensics team retrieved a blood

sample from the crime scene. ◆ They prepared DNA profiles of the

blood sample, the victim and a suspect as follows:

V S S1 S2 S3

V Victim

S Sample from crime scene

S1 Suspect 1

S2 Suspect 2

S3 Suspect 3

DNA profiles

Solving Medical Problems

DNA profiles can be used to determine whether a particular person is the parent of a child.

A childs paternity (father) and maternity(mother) can be determined.

This information can be used in • Paternity suits • Inheritance cases • Immigration cases

Example: A Paternity Test

◆ By comparing the DNA profile of a mother and her child it is possible to identify DNA fragments in the child which are absent from the mother and must therefore have been inherited from the biological father.

Starting position of sample

1 2 3 4

Genetic fingerprint of …1 mother

2 child

3 possible father A

4 possible father B

There is a match between one of the child’s restriction fragments and one of the mother’s.

There is also a match between the child’s other fragment and one from possible father A.

Neither of the child’s restriction fragments match those of possible father B

Paternity test

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B. Whose your daddy?Which sample is most likely to be the father?

F1 or F2

C. Identical or not?

Which sets of twins are identical twins?

A. Who done it?

Which suspect matches the bloodstain?

Famous cases◆ In 2002 Elizabeth

Hurley used DNA profiling to prove that Steve Bing was the father

of her child Damien

Famous Cases◆ Colin Pitchfork was

the first criminal caught based on DNA fingerprinting evidence.

◆ He was arrested in 1986 for the rape and murder of two girls and was sentenced in 1988.

Famous Cases◆ O.J. Simpson was

cleared of a double murder charge in 1994 which relied heavily on DNA evidence.

◆ This case highlighted lab difficulties.