DNA evidence is a modern criminal technology that can reliably prove the presence of suspects at the scene of a crime. The forms of DNA evidence include:
- Hair
- Saliva
- Blood
- Sperm
- Skin cells
- Other bodily fluids
If any of the above is found at the scene of the crime, it can be analyzed for DNA evidence and compared to DNA samples on file or a suspect. With the exception of identical twins, everybody has unique DNA, so DNA evidence is a highly accurate form of evidence.
The History of DNA Evidence In Criminal Cases
DNA was first used in the 1980s, but it wasn’t used in criminal cases until 1986. The first use of DNA evidence was by Dr Jeffries in two rape and murder cases in Leicestershire, UK. Fingerprints and DNA evidence recovered from semen stains at the crime scene led to a criminal conviction.
The first use of DNA evidence in the US was in the case of Tommie Lee Andrews, a Florida rapist. This led to his conviction and a 22-year prison sentence.
DNA is not just used in criminal cases; it has been popular for identity and paternity testing since the 1980s. The FBI were late to the party and did not adopt DNA testing until 1998. Since then, it has been used to reexamine old cases, either finding the perpetrator or freeing the wrongly convicted.
How Does DNA Evidence Work For Paternity Testing?
Each cell of our body contains DNA in its nucleus. It appears in the form of 46 chromosomes which are tightly coiled together to fit into the microscopic cell. The chromosomes are made up of 24 pairs, 22 of which are non-sex chromosomes and 2 of which are sex chromosomes. A male’s sex chromosomes will be XY, and a female’s sex chromosomes will be XX.
Our genes are made up of molecules which determine our physical characteristics. They are:
- Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C)
All of our cells have the same DNA coding.
How is DNA Extracted?
The DNA is housed in the middle of the cell, in its nucleus. When a scientist extracts DNA from a sample, they use robotic instruments to break open the cells and separate the DNA from other cell parts.
Polymerase Chain Reaction
Polymerase chain reaction uses the cells replicating ability to achieve a large sample for testing. This is achieved by using primers and enzymes.
Short Tandem Repeat Analysis
Short Tandem Repeat analysis looks at the 20 different loci to determine the familial relationships.
Genetic Analyzer
A genetic analyzer allows scientists to analyze 48 samples of DNA at one time. This equipment applies electrical current to the DNA through the capillary tube. The electrical current makes the smallest pieces get to the end of the tube first. At the end of the tube, the laser activates the fluorescent tags.
Paternity Test Report
All of the DNA information will be incorporated into a paternity test report. The report compares the alleles to see if there are matches between the pairs. The report will evaluate all of the STR locations to determine how close the DNA match is. Based on the percentage of the match, the report will give a percentage likelihood of familial relationship.
How Is DNA Evidence Used to Solve Crimes?
Aside from comparing DNA between family members, DNA evidence can be used to tie a person to a crime. Even if the DNA evidence is not a good sample, they can rule out suspects based on genetic features. For example, a DNA sample might tell scientists the sex of the person who committed the crime to help them narrow down their suspect pool.
DNA evidence will examine the following features of DNA:
- Short Tandem Repeats – Everyone has a unique combination of STR pairs; therefore it is easy to compare suspects STRs with DNA evidence found at the scene of the crime.
- Single Nucleotide Polymorphisms – These are letter changes to the DNA that can help track a person’s ancestry. It will tell scientists about family history and the migration of ancestry.
- Mitochondrial DNA – Mitochondria produce energy for the cell. The mitochondrial DNA is inherited from the biological mothers and therefore can determine someone’s maternal lineage. The mitochondrial DNA is used in scientific testing of remains found at archaeological sites. It can determine the maternal lineage of ancient royalty or any other remains.
- Y Chromosome Typing – The Y chromosome can be used to trace paternal lineage in men. In most cases, it will not change through the generations, making it easy to track people who share paternity. Scientists and historians have been able to use Y Chromosome typing to trace the early origins of early human populations. So far, they have been able to trace back 100,000 years. They can unlock migration patterns to track the movement of human populations through the generations.