Forensic DNA Fingerprinting

Profiling and Crime Scene Evidence

Dec 8, 2007 Karen Lotter

DNA plays an important role in modern forensic science. Today, DNA fingerprinting has become one of the primary methods of identifying people and solving crimes.

Definition of DNA

DNA stands for Deoxyribonucleic Acid. It is the genetic material of a cell. DNA is can also be described as the blueprint of an organism.

Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria, where it is called mitochondrial DNA or mtDNA.

Human DNA consists of about three billion bases and more than 99 percent of those bases are the same in all people. It is divided into functional units called genes which carry all the instructions for making up our body.

Where Is DNA Contained in the Human Body?

DNA is contained in blood, semen, skin cells, tissue, organs, muscle, brain cells, bone, teeth, hair, saliva, mucus, perspiration, fingernails, urine, feces, etc.

DNA is really a code, divided up into sections. Dr Matthew Hodgkin, a Research Fellow at Birmingham University's Institute of Cancer Studies explains: “You may have heard the term 'double helix'. This describes the DNA molecule. It is two strands coiled round each other. You inherit half your DNA from your mother and half from your father. The genes are all in pairs and which of each pair you inherit from each parent is randomly determined when the egg or sperm cells are developing. This explains why an inherited disease caused by one gene will only be passed on to children in 50% of cases."

How DNA Profiling Helps to Solve Crimes

DNA profiling or fingerprinting was developed in 1984 by British geneticist Sir Alec Jeffreys and first used in forensic science to convict Colin Pitchfork in the 1988 Enderby murders case.

A DNA fingerprint is the same for every cell, tissue and organ of a person. This DNA fingerprint cannot be altered by any known treatment.

It makes sense that DNA evidence has become such a powerful crime solving tool because no person's DNA fingerprint is the same except for identical twins. This means that DNA collected from a crime scene can either link a suspect to the evidence or eliminate them which is why accurate DNA fingerprinting is critical as a crime solving tool.

Crime Victims can be Identified and Crime Scenes can be Linked through DNA Profiles

Crime victims can be identified through DNA from relatives, even when no body can be found. And when DNA evidence from one crime scene is compared with DNA evidence from another, those crime scenes can be linked to the same perpetrator (in the USA) through systems like CODIS (Combined DNA Index System), an electronic database of DNA profiles that can identify suspects.

Collecting DNA Evidence at a Crime Scene

DNA evidence can be collected from virtually anywhere at a crime scene. DNA has helped solve many cases when imaginative investigators collected evidence from non-traditional sources:

A masked rapist was convicted of forced oral copulation when his victim's DNA fingerprint matched DNA evidence swabbed from him.
Many cases have been solved by DNA analysis of saliva on cigarette butts, postage stamps, rims of cups and glasses.
DNA analysis of a single hair found deep in the victim's throat provided a critical piece of evidence to solve the crime and apprehened the criminal.

How Does DNA Fingerprinting or Profiling Work?

The chemical structure of everyone's DNA is the same. The only difference between people is the order of the base pairs. There are so many millions of base pairs in each person's DNA that every person has a different sequence.

Using these sequences, every person could be identified solely by the sequence of their base pairs. Instead, scientists are able to use a shorter method due to repeating patterns in the DNA. These patterns do not, however, give an individual "fingerprint," but they are able to determine whether two DNA samples are from the same person, related people, or non-related people. This is called DNA fingerprinting or profiling

* Read the story of Professor Sir Alec Jeffreys, University of Leicester, inventor of DNA Fingerprinting or profiling.

Sources:

[PDF], What Every Law Enforcement Officer Should know about DNA

What is DNA? What does it stand for?

The copyright of the article Forensic DNA Fingerprinting in Forensic Science is owned by Karen Lotter. Permission to republish Forensic DNA Fingerprinting in print or online must be granted by the author in writing.