How is DNA Fingerprinting Used to Crack Criminal Cases?

Deoxyribonucleic acid, or DNA, was first identified in the late 19th century by Swiss chemist Friedrich Miescher, who initially referred to it as “nuclein.” It wasn’t until the mid-20th century that James Watson and Francis Crick unraveled the double helix structure of DNA, with essential contributions from Rosalind Franklin’s X-ray diffraction images. This groundbreaking discovery in 1953 set the stage for the modern field of genetics and opened up a deeper understanding of how genetic information is encoded, replicated, and inherited.

DNA fingerprinting, also known as DNA profiling, emerged in the mid-1980s as a transformative tool in forensic science. The technique was pioneered by Sir Alec Jeffreys, a geneticist at the University of Leicester, who discovered that specific regions of DNA vary greatly among individuals, much like fingerprints. This realization led to the development of DNA fingerprinting, allowing for the identification of individuals based on their unique genetic makeup. DNA fingerprinting revolutionized criminal investigations and legal proceedings, providing a powerful method for solving crimes, establishing paternity, and exonerating the wrongly accused.

DNA fingerprinting works by analyzing specific regions of the genome. Labs will process and extract the DNA and then amplify it to read the patterns. Then, the DNA samples are compared to see if they match. This process works because everyone, except identical twins, has unique DNA.

“We are looking at variations in DNA between individuals. We’re looking at the areas of DNA that are variable between individuals that give us a unique fingerprint or profile that could be used to identify an individual,” explains Dr. Michael Marciano, professor of practice and director of forensics research at Syracuse University.

This field has been around for decades, and while many things have remained the same, there have also been some incredible advancements. “One thing about forensic science is that it is always changing. However, one misconception about forensic science is that we’re not state-of-the-art. The technologies we’re looking at in forensic science have been in the mainstream for decades because we have to show incredibly high reliability and go through the court system. So many of these methods we will use will not be the newest methods because we have tried-and-true methods,” explains Dr. Marciano. “That being said, we never stop innovating in forensics—it just takes a little longer.”

Keep reading to learn more from Dr. Marciano about how DNA fingerprinting works, its applications outside of criminal cases, the challenges of collecting DNA samples, the accuracy of this technology, and how the DNA database works.

Meet the Expert: Michael Marciano, PhD

Dr. Michael Marciano is a professor of practice and director of forensics research at Syracuse University, where he specializes in forensic science, focusing on genetic identity and DNA analysis. He holds a PhD in structural biology, biochemistry, and biophysics from Syracuse University, a master’s of science in forensic molecular biology from the State University of New York at Albany, and a bachelor’s of arts in biology from the University of Rochester.

Dr. Marciano’s research intersects forensic science and national security, utilizing advanced laboratory techniques and machine learning to analyze complex DNA samples. His professional background includes significant contributions to forensic science practice and research, with numerous publications and patents in the field. He is also a committee member for the New York State Commission on Forensic Science and the Organization of Scientific Area Committees (OSAC) – Human Forensic Biology.

How DNA Fingerprinting Works

DNA fingerprinting analyzes specific genome regions known as short tandem repeats (STRs). These sections of DNA vary among individuals and can be used to identify an individual uniquely.

The first step in DNA fingerprinting involves obtaining a sample of DNA, which can come from sources such as hair, blood, saliva, or skin cells. Once the sample is collected, it is processed and amplified to read the patterns within the STRs. “We release the DNA from the cell, and then we use a technique called polymerase chain reaction (PCR) to create millions of copies of the selected DNA regions. This allows us to take a very small amount of DNA and make enough to then be able to detect it with instrumentation,” explains Dr. Marciano.

These amplified DNA fragments are then separated and visualized through a process called capillary electrophoresis. By comparing the pattern of bands formed on the gel to those from known samples, scientists can determine the likelihood of a match between DNA samples, thereby establishing individual identity with a high degree of probability. “The bands of DNA are actually detected peaks, and that is what our DNA profile is. The DNA profile is composed of different DNA locations identified as variable between individuals,” says Marciano. “In mainstream criminal forensics, we don’t look at any disease-causing genes or any regions of the DNA that can tell us what a person looks like.”

Applications Outside of Criminal Prosecution

While DNA fingerprinting is most commonly associated with criminal investigations, the technique has also been utilized in other areas. “There are many applications outside of criminal forensics. One is with unidentified human remains. Say we find skeletal remains. Who was it? They might not have a criminal history, but we can use DNA fingerprinting to identify them if we have a sample to compare to,” says Dr. Marciano. “Then there’s also whole genome sequencing, which is a burgeoning field where we use a huge amount of DNA sequence data to help identify identify relatives, for example. Genetic genealogy relies on many of the techniques we use in forensic science.”

Another area where DNA sequencing is critical is with mass casualties. “The fires in Hawaii, for example, are a great example of noncriminal use of DNA fingerprinting. Same with the World Trade Center. Sure, the attack was a criminal act, but the identification was looking at the victims in this case,” says Dr. Marciano.

Accuracy of DNA Fingerprinting

DNA fingerprinting is renowned for its high level of accuracy in identifying individuals. This precision stems from examining specific regions of the DNA that are highly variable among individuals, known as short tandem repeats (STRs). The likelihood of two unrelated individuals having the same DNA profile is astronomically low. “Accuracy can be tricky because it all depends on the amount of data you have and how unique it is. However, it’s safe to say that a single source profile is unique in the human population unless they’re identical twins,” notes Dr. Marciano.

He continues, “We have to be very careful how we say this, though, so what we explain is that we expect you to find a profile once in over a quadrillion individuals. So when we used to use this reporting method, we would say in court that we found a DNA profile at the crime scene, and this DNA profile matches this individual,” he articulates.

Challenges in Collecting and Analyzing Samples for DNA Fingerprinting

Collecting DNA samples presents several challenges that can affect the quality and reliability of the results. “One of the main challenges is not having enough. Forensic samples are not tidy like samples collected and kept in labs. They are in puddles in parking lots, they have gas in them, they’ve been exposed to the elements, et cetera,” says Dr. Marciano. “So getting enough DNA is often very difficult.

Another challenge is that rarely is a sample limited to one person. “A lot of the samples are mixtures of individuals. Picture a crime in a convenience store. Maybe 100 people use that door handle to enter the store, or the money that the person touched and dropped has had 50 people touch it. So a challenge is being able to interpret mixtures of samples, mixtures of individuals, and try to separate them, almost like a puzzle,” he shares.

How The DNA Database is Used (CODIS)

CODIS, or the Combined DNA Index System, is a pivotal tool used in forensic science to match DNA profiles. Developed by the Federal Bureau of Investigation (FBI), CODIS allows laboratories to exchange and compare DNA profiles electronically, thereby aiding in identifying individuals involved in criminal cases. Comprising numerous local, state, and national databases, CODIS enables storing and comparing DNA profiles obtained from crime scene evidence, convicted offenders, and missing persons. This comprehensive system helps link repeat offenders to unsolved crimes and assists law enforcement agencies in quickly and accurately identifying individuals across multiple jurisdictions.

The databases within CODIS are strictly regulated to ensure the privacy and security of the information contained within them, adhering to stringent legal and ethical standards.

“The DNA database is one of the areas the public has the greatest misconceptions,” says Dr. Marciano. “It comprises samples from convicted offenders, forensic unknowns, and unidentified human remains and missing persons. In many states, you could be required to give a DNA sample if you’re arrested. In some states, it is if you are arrested and convicted of a felony. There are complex flow charts on entering a profile that ensure the information is accurate and secure.”

The information in the database is constantly changing, depending on state, local, and federal laws. “New York State, for example, just decriminalized marijuana. So what happened was anyone in the DNA database for marijuana charges had their DNA expunged,” he explains. “There are very strict guidelines for the data to be added, removed, and managed. I feel much more comfortable about my DNA if it were in the CODIS database than with a private company like 23andMe.”