Alec Jeffreys, the inventor of DNA analysis, was born on January 9, 1950, in Oxford, England. As a young man, he attended the University of Oxford, and in 1977, he began working at the biological labs at the University of Leicester. It was at Leicester in 1984 that he was able to separate DNA fragments into recognizable patterns and record them using an X-ray machine. This process, called restriction fragment length polymorphism (RFLP), remained the standard method of DNA analysis for several years.

Having made significant scientific discoveries, Jeffreys was elected to a prestigious scientific organization, the Royal Society, in 1986, and in 1994, the English queen knighted him, making him Sir Alec Jeffreys. He also won the Albert Einstein World Award of Science in 1996 and the Copley Medal of the Royal Society in 2014, which is awarded for outstanding achievements in science.

Why DNA Testing Matters

By “forensic DNA typing,” Jeffreys meant a way of using DNA in the same way detectives used fingerprints to seek out and arrest criminals. It makes sense, then, that he coined the term “genetic fingerprinting,” which became a process now commonly called DNA profiling. This new technology seemed to possess a number of distinct and remarkable advantages for investigators trying to identify suspects in criminal cases. First, the chances of any two people having identical DNA, especially in the variable region of minisatellites, is extremely tiny. Therefore, DNA tests produce a genetic snapshot that can single out one suspect from all other suspects. Former prosecutor Harlan Levy explained the mathematical probabilities involved:

It may be that a DNA fragment [group of minisatellites] in one sample that could be expected to occur in one person in 100 ... matches a fragment in [another] sample. That is significant but hardly overwhelming. But if there is a DNA match at another location [on the DNA helix taken from the samples], the numbers suddenly grow exponentially . The chances of the two DNA profiles matching randomly are one in 10,000 . If there is a match at a third location ... the numbers would go to 1 million. And if there is a fourth match ... it goes to one in [many millions].9

Similarly, if a suspect has seven, eight, or nine matches, the chances that they are not the culprit is one in hundreds of billions or even trillions. Since there are more than 7.5 billion people on Earth, it is extremely likely that the suspect is the guilty party.

Additionally, Jeffreys and his colleagues soon realized a second advantage of DNA profiling: Nearly every cell in the human body has a nucleus that contains DNA. This means that almost any cell from a suspect, such as a skin cell, hair cell, saliva cell, or urine cell, carries the suspect’s genetic fingerprint. This vastly expands the diversity of trace evidence—tiny amounts of evidence, such as a single hair—that might be found at a crime scene and analyzed, thereby increasing the likelihood of identifying the criminal. It also increases analysts’ ability to match trace evidence found at the scene of the crime with the person who committed the crime. Because the same DNA can be found in every cell with a nucleus, analysts can take a sample of saliva from a suspect to match with skin or hair cells found at the crime scene; they do not have to take samples from the same part of the body to get a match. “Fingerprints come only from fingers,” researcher Ngaire E. Genge wrote:

But DNA can be found in blood, in urine, in feces, in saliva, in some hair, in the shed skin cells found in a facecloth or toothbrush—even in the sweatband of a hat! A suspect doesn’t have to bleed at the [crime] scene to leave DNA. Semen at rape scenes, saliva on the envelope of a ransom note, skin cells scraped onto a rope while tying a victim— all provide the opportunity for collection and analysis.¹⁰

A third advantage of DNA collection and analysis is that DNA can survive much longer than most other kinds of evidence. Most fingerprints smudge or disappear after a few weeks or months (although on occasion they can last a few years). In comparison, DNA can, under the right circumstances, last for centu-ries—or even for millennia. Successful DNA analysis has been performed on Egyptian mummies up to 4,000 years old, and similar tests have been conducted on 25,000-year-old human remains found in a cave in southern Italy.

The remarkable longevity of DNA evidence allows investigators to go back and look at cold cases. These are criminal cases from 5, 10, 20, or even 50 years ago that were never solved, often due to lack of compelling evidence. If any evidence found at the crime scene is still in storage, it can be tested, and this could potentially solve the case. The culprit might be caught and jailed. If this is not possible, at least the victim’s family can achieve some kind of closure.

DNA can be found in all kinds of places, including hair follicles (although it cannot be found in hair shafts).

Yet another advantage of DNA is its ability to indicate familial, or family, relationships. Because each person inherits some DNA from each parent, their DNA is very similar to, though not exactly the same as, their parents’. A person’s DNA is also similar to their siblings’ DNA, and, to a lesser degree, to any aunts, uncles, and cousins.

A murder case that occurred in the Philippines illustrated how this can be helpful to crime investigators. Two people committed the murder, but only one was identified by an eyewitness and arrested. Some saliva found at the crime scene was analyzed, and the DNA it contained was very similar, but not an exact match, to the DNA of the man in custody; this showed that the second killer was almost certainly one of this man’s close relatives. Based on this, police questioned the man’s brother, who soon confessed to his role in the crime.

Unleashing DNAs Potential

The case of the two brothers was not the first to use DNA samples to solve a murder. That distinction went to two related cases that occurred in England in the 1980s, which Jeffreys was instrumental in solving. The first murder took place in November 1983. A 15-year-old schoolgirl named Lynda Mann went missing in the village of Narborough, only 6 miles (9.7 km) from Jeffreys’s lab. The next morning, someone found the girl’s body on a footpath in the local woods. The coroner—an official who examines dead bodies—determined that she had been strangled to death. There were also semen stains on her body, indicating that a rape had occurred. Unfortunately, no other evidence was found—no fingerprints, clothing fibers, or forgotten items. The semen was all the police had to go on. However, because DNA profiling did not yet exist, there was no way to tell whose semen it was, so the case could not be solved.

The situation changed when, three years later in July 1986, 15-year-old Dawn Ashworth was murdered in the same village. Two days after she was reported missing, police found her body. Like Mann, she had been strangled and raped. Police arrested Richard Buckland, who worked at a nearby hospital. Witnesses had seen a man matching his description near the scene of the murder a few days before, and Buckland had knowledge of unreleased details about Ashworth’s murder.

Under intense questioning, he finally confessed that he had raped and strangled Ashworth. Due to similarities in the methods and details of the Ashworth and Mann murders, the police suspected that Buckland had committed both crimes. However, Buckland continued to deny that he had committed the earlier crime.

One of the detectives remembered reading about the recent discovery of DNA analysis. Hoping for a break in the case, the police delivered a sample of the semen found on Mann and a sample of Buckland’s blood to Jeffreys, who analyzed the DNA in both samples. The DNA of Mann’s killer did not match Buckland’s DNA, so Buckland could not be the murderer in that case—but even more surprising were the results when Jeffreys analyzed the semen found on Ashworth and compared it to Buckland’s DNA: There was no match. This meant Buckland had not killed either girl and had falsely confessed to the second murder. The police saw no other choice but to release him from custody. “I have no doubt whatsoever,” Jeffreys later said, “that he would have been found guilty had it not been for DNA evidence. That was a remarkable occurrence.”11

The police were disappointed. Rather than solving two crimes with one killer, they suddenly had no suspects for either murder and had to start from scratch on both cases. However, this time, they were armed with the new DNA technology. Early in 1987, they began testing blood and semen samples from every man they could find in Narborough and surrounding villages. They did not find a match until they learned that a local bakery worker, Colin Pitchfork, had paid another man to submit his own blood and claim it was Pitchfork’s.

DNA samples collected at crime scenes are brought to labs, where they are analyzed.

The police arrested Pitchfork in September, and he confessed to both murders. Jeffreys then administered new DNA tests to confirm the confession, and the tests showed that Pitchfork’s DNA exactly matched the DNA in the semen found on both dead girls. According to Harlan Levy,

It was an extraordinarily auspicious [promising] beginning for the new technology. In its first known application in a murder case, DNA testing fulfilled both its promises, clearing an innocent man and helping convict a guilty one ... DNA testing was completely new, but there was no doubt about its potential within the criminal justice system. Prosecutors and the press rushed to embrace the novel tool.¹²

Today, that tool is a standard and vital one used by law enforcement officials across the United States and in many other nations.

The process of DNA profiling involves a number of steps that must be taken in a specific order. Some of them are complex, and all of them require considerable skill and deliberate care. CSIs and other experts who work with DNA samples must be highly educated and specially trained. These investigators and lab technicians generally have college degrees in biology, chemistry, genetics, forensic science, or some related scientific field. Moreover, some crime labs will only hire a DNA specialist who has a minimum of six months to one year of on-the-job experience. This experience can be learned in a noncriminal setting; for example, some hospitals hire DNA analysts to test for genetic diseases.

All this training is necessary because it is vital that samples are handled with extreme care during every step of the profiling process. It is very easy to contaminate a sample. A DNA sample that has been contaminated or mishandled cannot be used as evidence in a court of law. If the sample was one that was taken from the crime scene, there is no way to get another one, and the suspect may be set free even if they are guilty. In the same way, contamination of a DNA sample might result in someone who was mistakenly sent to jail losing their only chance to be freed.

The steps in the DNA profiling process used by these dedicated specialists can be grouped into two general categories: collecting evidence at a crime scene and analyzing the samples back at the lab. When collecting samples, investigators follow strict guidelines designed to ensure that nothing is missed and that all evidence is properly handled. At the lab, technicians follow a standardized procedure to ensure that results are permissible in court.

Where Can DNA Be Found?

At a crime scene, police often use bright yellow tape to mark the area that needs to be inspected by officials and to protect it from passers-by. The fewer people who interact with the crime scene before investigators arrive, the better; in this way, police can ensure that any evidence collected will be relevant to the case and will remain uncontaminated.

The most common sources of DNA evidence are blood, semen, saliva, and skin. Skin cells can be found in multiple ways, including if the culprit has dandruff or was injured while committing the crime. Red blood cells have no nuclei and therefore do not contain DNA; however, white blood cells do contain DNA, so when CSIs collect blood samples, they analyze the white blood cells to build a DNA profile. The same is true of hair. Although hairs are often found at crime scenes, the cells in the shaft, or main part of the hair, have no DNA. However, if a hair has been pulled out by the root—which can happen if the victim fights back against their attacker— DNA can be found in the skin cells on the root.

Bones, teeth, urine, and feces also contain DNA. These are less commonly found at crime scenes, but they can help investigators in other ways. For example, dental records and DNA taken from the victim’s teeth can help investigators identify a body.

To find these vehicles for DNA, investigators begin looking in a number of obvious places at a crime scene. Saliva, for example, is often found on items that typically interact with the mouth, such as the rims of cups and glasses, silverware, toothbrushes, telephone receivers, and cigarette butts. The most common places investigators find semen specimens are the genital areas of rape victims, bed sheets, and discarded condoms. Likewise, skin cells or blood from violent criminals are frequently found under the fingernails of victims who scratched them while resisting. Traces of incriminating blood are often discovered in the cracks between floorboards after a criminal has tried to clean up a crime scene.

DNA evidence can also turn up in places that are far less obvious and at times even surprising to untrained observers. For instance, valuable biological evidence can sometimes be found in the bathroom trash. A wastebasket can contain discarded tissues, cotton swabs, and other items containing traces of bodily cells or fluids. Similarly, contact lenses are typically coated with fluid from the wearer’s eyes, and envelopes that someone licked might still contain traces of that person’s saliva.

DNA can be found in many places; one of the most common is on the rims of recently used cups or mugs.

It might seem logical that urine samples would most likely be found in the bathroom, especially in, on, or around the toilet. Although this is often the case, experienced CSIs know they should also look elsewhere—especially when the crime scene is in a natural setting. According to one veteran investigator who collected DNA at crime scenes for 18 years,

After working a bunch of crimes that took us to cabins and other woodsy locations, we eventually came to the conclusion that, if you put ten guys in the woods and they’ve got to [urinate], nine of those ten guys will prefer to pee on a tree trunk ... shine a source light around a campsite and I’ll guarantee you there’ll be DNA from urine on some tree somewhere [in the area].13

Gathering the Evidence

When CSIs first arrive at a crime scene, they do not know which items and materials are going to produce DNA evidence that will solve the crime. They might find a used napkin or some stray hairs, for example, but the presence of someone’s DNA does not automatically make them a criminal. It might turn out that these belonged to the victim or an innocent bystander, not the suspect.

DNA makes up the genetic material of all animals, not just humans, which can occasionally help forensic scientists solve crimes by ruling out or convicting a human suspect. Forensic science researcher Ngaire E. Genge wrote about a case in which cat hair allowed police to convict a murderer:

One of the first cases linking two people through non-human DNA was investigated by the Royal Canadian Mounted Police (RCMP). While investigating a death in Prince Edward Island, examiners recovered two white hairs, which were at first thought to be those of the victim’s ex-husband. They weren’t. They were cat hairs. Not having a unit that dealt with cat DNA, the RCMP sent the hairs, as well as a blood sample from the ex-husband’s white cat, to the National Cancer Institute’s Cat Genome Project in Maryland. The lab there confirmed that both blood and fur came from the same cat, and the ex-husband was convicted of murder.1

1. Ngaire E. Genge, The Forensic Casebook: The Science of Crime Scene Investigation. New York, NY: Ballantine, 2002, p. 150.

For this reason, the guidelines for evidence collection require that a wide range of items and biological materials be collected at the crime scene. These can be sorted through and analyzed later at the lab to determine whether or not they are important to the case. The U.S. National Institute of Justice (a division of the U.S. Department of Justice in Washington, D.C.) issues a checklist of DNA-related items and materials that investigators should look for at a crime scene. The list includes fingernails, paper towels, tissues, Q-tips, toothpicks, straws, cigarette butts, blankets, sheets, mattresses, pillows, dirty laundry, eyeglasses, contact lenses, cell phones, ropes, stamped envelopes, used condoms, and more.

According to the collection guidelines, investigators must make sure they do not touch any of the evidence with their bare hands or fingers in order to prevent an investigator’s own cells and fluids from contaminating the evidence. The person doing the collecting should wear rubber gloves and change them often. For the same reason, investigators should be careful not to touch their face, hair, glasses, and so forth while wearing the gloves. All of these often thoughtless gestures can result in contamination of a DNA sample—for example, if an investigator touches their mouth and some of their saliva transfers onto the gloves they are using to collect samples, they may have made that sample unusable.

In addition to gloves, investigators are expected to use clean disposable items, such as cotton swabs, small wooden sticks, or wooden tweezers to pick up the evidence. For instance, an investigator will not use the same swab to collect two different samples; that way, one piece of evidence does not contaminate another.

Once they have picked up the evidence, the investigator places it in a clean paper evidence bag. This paper bag is important because airtight containers or plastic bags can create moisture in the container that can compromise the evidence. In addition, the evidence must be allowed time to dry before being sealed in a paper bag so it does not decompose. The bag must be clearly labeled, indicating both what the evidence is and where it was found. That helps eliminate mistakes later in the lab, such as accidentally mixing up two or more samples. A biohazard symbol and label must also be placed on the bag to warn people at a glance that it contains potentially dangerous material. For instance, diseases can be transmitted through a person’s blood or saliva, so care must be taken when handling this evidence.

Guidelines recommend that while handling and packaging evidence, investigators should try as hard as they can not to sneeze, cough, or even talk. All of these activities spray tiny saliva droplets. Though they are nearly invisible to the human eye, they can land on and contaminate the evidence. For this reason, many investigators wear surgical masks while handling evidence at a crime scene.

Evidence is not the only thing that must be protected at crime scenes. The investigators and lab technicians must also be careful when handling evidence that might cause them harm. CSIs, Genge pointed out, regularly handle a number of bodily fluids that could be carrying disease, germs, and other harmful biohazards: “Biohazards like hepatitis and HIV are real dangers to investigators and laboratory personnel who handle biowastes and bio-fluids, so all samples must be treated as infectious until proven otherwise.”14

Back at the Lab

All evidence samples found and collected at a crime scene are sent immediately to the nearest forensics lab. Once there, the first step is for technicians to go through each sample to make sure all the evidence is biological material that has the potential to contain DNA. Quite often, it is easy to identify where a sample might have come from; for example, a pool of dark red liquid lying beside a bullet hole or gash in a dead person’s head is almost certainly going to be blood. It is also generally safe to assume that yellow stains found on the rim of a toilet seat are urine.

However, investigators cannot always be sure what a substance really is until it is tested in the lab. In rare cases, for instance, a yellow stain on a toilet could be residue of a nonbiological liquid that someone spilled there. Similarly, hairs found at a crime scene might have neither human nor animal origins; instead, they might be synthetic (man-made) fibers from a wig. To avoid wrongly categorizing substances such as these, lab technicians run a brief preliminary test to identify it. This can save both time and money. As forensic DNA experts Norah Rudin and Keith Inman wrote, “It would be wasteful to run a full spectrum of DNA tests only to find no result because ketchup or shoe polish was analyzed.”15

Once all the evidence samples are satisfactorily identified, it is time to test the relevant ones for DNA. To do this, the DNA must be extracted, or separated from, the cells of the blood, semen, hair roots, bone marrow, or other biological material involved. The extraction process is fairly simple and, depending on the method used, can take anywhere from five minutes to two hours. The fastest method is not always the best; sometimes, to get the best quality DNA sample, a more time-consuming method needs to be used. For example, if a hair follicle has dirt on it, this may lower the quality of the results of a particular method. Regardless of the method used, the process follows certain steps:

1.    The cells must be separated from each other and then broken open so the DNA can be exposed.

2.    The DNA must be separated from the other material in the cell.

3.    Alcohol is added to the DNA to make it visible, so scientists can work with it more easily.

4.    The DNA is purified (cleaned) and suspended in a solution to keep it from disintegrating.

5.    The DNA is tested to determine its concentration and purity. The more concentrated and pure the sample is, the better results researchers will be able to get from future tests.