Forensic Deoxyribonucleic Acid: History and Applications

Paulean Gonzalez

Deoxyribonucleic acid (DNA) is located in the nucleus of every cell and is the genetic material that makes up cellular organism and viruses. However, DNA can be used in multiple ways when it comes to forensics. It assists in linking an individual to a criminal act, to exonerate victims or even to identify victims in mass disasters. DNA is what accounts for genetic information and DNA also codes for the proteins that are necessary for our body to survive.

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The history of DNA dates back to Gregor Mendel as well as Francis Crick and James Watson. DNA can be found in many different parts of the body but none of that would really matter if forensic units had no way of determining and linking the DNA to certain individuals which is why the multiple ways of DNA testing are so important. The inheritance of characteristics as well as the functions of DNA also plays an important role in forensic DNA. While what could be considered the most important part to how forensic DNA is used is the outcomes and the ways it was used in courts.

Gregor Mendel

It was in 1865 that Gregor Mendel, who was an Austrian monk, went before the Brno Natural Science Society and presented two lectures that summarized his experiment results on heredity in the garden pea. Mendel’s experiment was based off the cross pollination of a wrinkly green pea plant with a smooth yellow pea plant. In this experiment Mendel was able to discover that there was both a dominant and a recessive trait. His experiment allowed him to come up with three different conclusions. His three conclusions were that the inheritance of each trait was determined by what is known as a gene, that an individual receives one set of genes from each parent, and that even though a trait does not show up in an individual it is possible to still be carried on to the next generations. In Mendel’s experiment he discovered what is known as the Principle/Law of Segregation or otherwise known as his First Law. In this law there was four different parts that were included. The first part was that there were other forms of the genes that were inheritable known as alleles. The second part was that each offspring receives one allele from each parent. The third part both the sperm and the egg hold one allele for each trait and during fertilization they pair. Lastly, if the alleles are different only one appears while the other is not. The one that is shown is the dominant trait while the one that is not shown is the recessive trait. Mendel also came up with what is known as the Principle/Law of Independent Assortment which is also known as Mendel’s Second Law. In this principle, Mendel discovered that the different alleles were passed on individually and not based on one other. Mendel saw many different combinations which meant that there was separation from one another. In the early 1900’s it was believed that inheritance was fluid in nature, which was termed “half-blood” and “true-blood.” However due to Mendel’s experiment and his repeat experiments, it was discovered that the genetic information was a particulate, which is the “unchanging nature of the DNA molecule that allows DNA fingerprinting” (Herrero, 2009, p. ).

Crick and Watson

Roughly around 50 years ago Francis Crick, an English graduate student, and James Watson, an American post doctorate researcher, first proved that the structure of DNA was a double helix. For their efforts Crick, Watson, and Maurice Wilkins were awarded the 1962 Nobel Prize in Medicine for their “discoveries concerning the molecular structure of nucleic acid and its significance for information transfer in biological material” (Herrero, 2009, p. ). Both Watson and Crick have helped in the basic understanding of manipulating DNA as well as the understanding of DNA replication, transcription, and translation. They believed that DNA was shaped like twisted ladder. The sugar phosphate backbone made up the sides of the ladders while the nitrogenous bases made up the rungs of the ladder. The ladder of DNA is made up of building blocks that are known as nucleotides. DNA makes up every chromosome and gene and is a polymer of repeating units that are known as nucleotides. Each of the nucleotides contains three specific parts; the phosphate group, sugar, and the nitrogenous base. The nitrogenous base consists of one of the four molecules including adenine, guanine, thymine, and cytosine (Herrero, 2009).

Types of DNA

There are also two types of DNA’s. The two types of DNA’s are the nuclear or chromosomal DNA which is inherited from mother and father and the mitochondrial DNA which is inherited from only the mother. DNA can be located in the cells of our body.

Nuclear DNA can be present in a cell’s nucleus and is a combination of information that is inherited from both parents. It helps in the makeup of an individual’s genetic material. Nuclear DNA is best known as the “molecule of life and contains the genetic instructions for the development of all living organisms” (Nuclear DNA, n.d). Nuclear DNA can be found in almost every single cell except for red blood cells. Nuclear DNA was clearly expressed in Gregor Mendel’s Pea Experiment. His experiment was able to present information to back up the statement that half the information that was received was from the mother while the other half of information was received from the father.

Mitochondrial DNA does not come from cell nucleus; it instead is located in the mitochondria of the cell. The mitochondrion is a “specialized subunit within a cell that functions as the powerhouse of the cell” (Herrero, 2009, p. ). It is more likely that a small sample of mitochondrial DNA could be discovered compared to nuclear DNA. This is because mitochondrial DNA is present in hundreds to thousands of copies in each cell compared to the only two nuclear DNA copies that are present in a cell. This means that all muscle, bone, hair, skin and many other body fluids are capable of finding mitochondrial DNA. The advantages to using mitochondrial DNA are that they are more sensitive which means less DNA is needed, degrades slower than nuclear DNA, and it can be used in cases where nuclear DNA cannot. The disadvantages to mitochondrial DNA are that all people of same maternal line will be indistinguishable and there is more work to be done, it is more time consuming, and it is more costly (Herrero, 2009).

DNA Tests

The two previous methods that were used were the Restriction Fragment Length Polymorphism (RFLP) Analysis and the Polymerase Chain Reaction (PCR). While the new method that is currently being used is the Short Tandem Repeat (STR). Although all tests may be different, they all require that an extraction of DNA from the sample. All three tests have had their fair share of helping the forensic units as well as being the go-to test that was responsible for the prosecution of many felons as well as the exoneration of many former criminals.

Restriction Fragment Length Polymorphism

RFLP was the first DNA profiling technique that was used and was seen in a widespread of areas. It was developed by Sir Alec Jeffreys in 1984. This test was capable of allowing the fragments of DNA to be measured. However as it became difficult and expensive as well as taking weeks to perform and being very demanding it became outdated. For this test analysts linked the size of fragments from a reference that was known to a crime scene sample that would allow them to match two DNA profiles. One thing that caused problems for this type of testing was that many samples from the crime scenes appeared to be too small to be tested. For example a speck of blood would be of no use with the RFLP testing. Due to biotechnological advances there would soon be more sensitive tests that would turn into what is today’s DNA profiling (Herrero, 2009).

Polymerase Chain Reaction

The PCR test was invented so that multiple copies of a small DNA could be replicated. This test was created in 1987 by Kary Mullis, a biochemist, and Henry Elrich, a nuclear biologist. The way that this test works is that an enzyme was found that could withstand the het needed to “unzip the two strands of the DNA double helix for replication without breaking apart the enzymes itself” (Herrero, 2009, p. ). A problem that regularly occurs with the PCR test can be contamination. When the samples are being amplified it is crucial to be very careful and avoid adding extra cells onto the sample before it is amplified. A simple sneeze to a laugh could ruin the evidence (Herrero, 2009)

Short Tandem Repeat

By 1998, the FBI created a more multiplex version of the PCR test known as the Short Tandem Repeat (STR). This test is still being used today. Similar to the PCR test, the STR test is capable of working with small samples. There are three different color dyes that are used to “distinguish STR alleles with overlapping size ranges” (Herrero, 2009). The STR test evaluates specific regions within nuclear DNA. It is color coded as well as automated and computerized which makes it so easy to navigate (Herrero, 2009).

Combined DNA Index System

In 1998 the Federal Bureau of Investigation (FBI) launched the Combined DNA Index System (CODIS). Today, 44 of the 50 states are allowed to collect DNA from all felons, 28 of the 50 states are allowed to collect DNA from juvenile offenders and 39 of the 50 states can collect DNA from those who commit certain misdemeanors. In 1994, Congress came up with the DNA Identification Act of 1994 which authorized the FBI to maintain a national database that allowed the sharing of DNA information between states. There are three tiers to CODIS, which are the local (LDIS), state (SDIS), and national (NDIS).

CODIS uses 13 different DNA regions that can vary from person to person and matches are searched for at more than one location on a genome for more accurate results. By 2004, all 50 states were connected with the limited profiles of those who had been convicted of serious, violent crimes. On October 30, 2004, President George W. Bush signed the Justice For All Act that expanded the CODIS system and allowed the collection of DNA from all federal felons and allowed the states to upload the profiles of anyone who had been convicted of a crime (Herrero, 2009)

Inheritance of Characteristics

The many different characteristics that we inherit are in the form of DNA. However, we do not inherit the characteristic; we simply inherit the information that produces our characteristics. Half of the genetic material that we inherit is received by each parent. The sperm and egg are produced in the gonads of the parents and results in the “production of gametes that carry only half of the DNA that made the parents unique” (Herrero, 2009, p. ). A new individual is created when the sperm and egg are bonded. Much like the parents this new individual has two copies of all the genetic information and can produce eggs that will only have one copy of each gene if a female; however if it is a male it also has two copies of all the genetic information but can only pass one of the two to each of his offspring. The information that we receive is in pairs. The reason for this as stated before is because half the information is received from the mother while the other half of information is received from the father (Herrero, 2009).

DNA in Courts

Due to fact that DNA can be found in blood, semen, saliva, urine, hair, teeth, bone and tissues, it plays an everyday role in courts.

Kirk Bloodworth

On March of 1985, Kirk Bloodworth had been convicted of the killing as well as sexual assault of a little nine year old. The little girl’s body was found dead in July of 1984. It had been discovered that she had been beaten with a rock, strangled and raped. Bloodworth was arrested based off an eye witness stating that they had seen him with the little girl earlier on the day that she went missing. Five eyewitnesses were even able to identify Kirk Bloodworth based off sketches. Other evidence that was presented against Bloodworth was that on the day of the incident he had told his wife that he had done something that would change their lives forever as well as him mentioning something about a bloody rock (Know the Cases-Kirk Bloodworth, n.d.).

Bloodworth appealed the decision. His reason for appealing was that the bloody rock had only been mentioned because during interrogation the police had shown him a bloody rock. The incident in which he told his wife their lives would change forever had been dealing with him forgetting to buy the food she had requested. The police also did not inform the defense that there was a possibility that there was another suspect. The appellate court decision led to Bloodworth being convicted and also sentenced to two life terms that would run consecutively (Know the Cases-Kirk Bloodworth, n.d.)

“In 1992, the prosecution agreed to DNA testing to be performed by the Forensic Science Associates” (Know the Cases-Kirk Bloodworth, n.d., p. 1). The victim’s shorts and underwear, a stick that been discovered at the scene, as well as an autopsy slide had been compared to Bloodworth. The PCR testing that was used was able to determine that the evidence left on the underwear did not match with Bloodworth. The same tests were then performed again by the Federal Bureau of Investigation and the same results were found (Know the Cases-Kirk Bloodworth, n.d.).

In June of 1993, Bloodworth was released from prison and in December of 1992 he was pardoned. Over eight years of his life were lived in prison with two of those years him facing execution. Bloodworth became the first person to be “exonerated from death row through post-conviction DNA testing” (Know the Cases-Kirk Bloodworth, n.d., p. 1). Kirk Bloodworth’s case set the stage for many cases that are soon to come (Know the Cases-Kirk Bloodworth, n.d.)

Freddie Peacock

After serving five years in a prison in New York and nearly three decades after being released on parole, Freddie Peacock was able to clear his name thanks to DNA testing. Peacock was the 250th person cleared through DNA testing after being convicted for crime in which he did not commit (Know the Cases-Freddie Peacock, n.d.).

The crime in which he had been serving had occurred on a night in July of 1976. A New York woman was returning to her apartment from work and was attacked. When the woman was unlocking her apartment door, the man approached her from behind, took her keys and threw her to the ground. Where she was thrown to the grown she struck her head. The woman was told that if she screams she would be killed. The woman was pulled by her perpetrator to the side of a house that was nearby and was raped. Once the perpetrator was done he returned the woman her keys and left. The woman went back to her apartment building where she told the building superintendent who then proceeded to call the police. She later testified that she was only able to see her perpetrators face when she was in a dark are. She was able to describe that her perpetrator was an African-American man who weighed about 150 pounds and had been wearing a white, flower shirt (Know the Cases-Freddie Peacock, n.d.).

The woman had originally struggled to remember the details of the crime but later admitted that she believed that her neighbor was the perpetrator. The superintendent responded by asking if it was Freddie and she had said yes. Peacock’s photo was included in a 10-photo collection where she identified him again. Two hours after the attack, Peacock was arrested. The woman once again identified him through a window in one-person show up procedure. After being interrogated for two and a half hours, he had initially denied being the perpetrator but the police claimed he confessed. Peacock confessed to having several severe mental illnesses and was hospitalized for it multiple times. Peacock was unable to tell authorities how, when, and where was raped. Peacock never signed the alleged confession that the officer wrote with all the details to the crime (Know the Cases-Freddie Peacock, n.d.).

At the trial, the victim testified saying that she knew he was a perpetrator because of his beard and eyes and claimed that deep down she knew it was him. She claimed that Peacock and she had only spoken twice and he had entered her apartment once uninvited. A doctor even testified that the bodily samples that were collected from the victim at the hospital, but other evidence was not presented. Peacock was eventually convicted and sentenced to up to 20 years in prison (Know the Cases-Freddie Peacock, n.d.).

Peacock had been released on parole after five years in the New York prison. Thanks to his families and church support he was able to receive treatment for his mental illness. After contacting The Innocence Project in 2002 to help him clear his name. DNA evidence was obtained and able to rule him out. This evidence was taken before a judge. A state judge tossed out the conviction. This meant his name had been cleared after 34 years of wrongful convictions (Know the Cases-Freddie Peacock, n.d.).


All types of cells in the body contain the copy of the same DNA, for example DNA can be found in the blood cells, saliva cells, tissues cells and even the semen cells.

DNA Functions

DNA has two primary functions. These two functions include transmitting information from one generation to the next as well as providing the blueprint for making proteins the same way every time.


Herrero, S. (2009). Forensic DNA: Technology, Application, and the Law. In S. H. James & J. J. Nordby (Eds.), Forensic Science: An Introduction to Scientific and Investigative Techniques (pp. 303-325). FL: CRC Press.

Know the Cases-Freddie Peacock (n.d.). The Innocence Project. Retrieved from

Know the Cases-Kirk Bloodworth (n.d.). The Innocence Project. Retrieved from

Nuclear DNA (n.d.). Retrieved from

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