DNA & Forensic Science


(Photo from Science Magazine)

The Innocence Project was founded in 1992 on the idea that if DNA technology could prove people guilty of crimes, it could also prove wrongfully convicted people are innocent. There have been 375 DNA exonerations to date; those innocent people spent 5,284 years wrongfully imprisoned.

Deoxyribonucleic Acid (DNA) is a substance that contains all genetic material and is found in every living cell. Every individual, aside from identical twins, possesses DNA that is completely unique to them. Unlike blood evidence and other commonly used types of forensic evidence, DNA that has not deteriorated is useful for an endless period of time.

DNA is used in criminal investigations to identify suspects and inform criminal convictions. DNA is also used to exonerate people who were convicted prior to the invention of modern DNA testing or in cases where DNA testing was not conducted or conducted improperly. There are two types of DNA: nDNA and mtDNA. 

DNA analyses traditionally utilize nuclear DNA (nDNA), which is extracted from the nucleus of a cell. Every cell in a person’s body contains a nucleus, which contains DNA that is entirely unique to the individual. Because nDNA is unique, analysts are able to conclude that two samples are consistent with each other. 

nDNA can tell you more information than mtDNA; however, it is harder to extract due to its location in the cell. Each cell has a nucleus, but if the nucleus is damaged, it can be difficult to retrieve nDNA. 

Mitochondrial DNA (mtDNA) is extracted from the mitochondria that surround the nucleus in each cell. Each cell contains up to 2,000 mitochondria, making mtDNA far more abundant than nDNA.

When forensic analysts compare mtDNA samples for a criminal investigation, there are two possible conclusions: (1) the samples are different, allowing the analyst to exclude the sample, or (2) the samples are consistent with each other. If this is the case, it is possible that the samples are from the same person, but because of the limits of mtDNA, the analyst may only conclude the samples are related

It is impossible to link mtDNA to only one individual because mtDNA is inherited from mothers. Thus, all people on the maternal side of the family will share the same mtDNA. This means that if an analyst determines mtDNA samples are consistent, the only thing they know for sure is that the samples are maternally related.

Christopher Tapp 

Christopher Tapp as a teenager before he was wrongfully convicted
Christopher Tapp as a teenager before he was wrongfully convicted (Photo from The Innocence Project)

 In 1996, 18-year-old Angie Dodge missed her shift at a local beauty supply store. A concerned co-worker stopped by Dodge’s apartment to check on her and discovered her lifeless and half-naked body on the bedroom floor. 

There were no leads in the rape and murder for six months. Then, in a different state, Doge’s acquaintance, Ben Hobbs, was arrested for raping a woman at knifepoint. When questioned in relation to Doge’s rape and murder, Hobbs implicated 20-year-old high school drop-out, Christopher Tapp. Tapp did not know Dodge but lived in the same small town as her.

DNA was extracted from semen at the crime scene, and Tapp was excluded as a contributor. Nonetheless, police questioned Tapp for more than 30 hours, utilizing tactics proven to generate false confessions. Among those tactics, the police fed Tapp information about the crime. The following is an exchange with investigators during Tapp’s interrogation. 

Investigator: “You hold her down while she’s being cut. You hold her down while she’s being?” 

Tapps: “Cut.” 

A jury relied on this false confession to convict Tapp of Dodge’s rape and murder, and he was sentenced to life in prison with a minimum of 30 years. 

In 2017, Tapp’s rape conviction was vacated based on additional DNA testing, but he was re-sentenced for the murder. The theory was that Tapp did not act alone, which was consistent with the story told in his false confession. 

In May 2019, analysts compared the DNA from the semen to an ancestral database and found it to be consistent with a sample submitted by Brian Dripps. Dripps’ name was familiar to the investigation because, as Dodge’s across-the-street neighbor at the time, he was questioned by the police in 1998 but offered no valuable information. Dripps confessed to the rape and murder and confirmed that he acted alone. Tapp was exonerated in July 2019. This was the first exoneration to rely on genealogical DNA testing.

Forensic Science:

Forensic science is the application of sciences to criminal investigation and law. Forensic scientists are often called as expert witnesses to testify to scientific conclusions based on evidence from crime scenes. There is a history of inaccuracy in forensic science. Some forensic sciences have been deemed unreliable while others have been completely discounted including fire investigation, bite mark evidence, and lead bullet analysis.

For example, on April 20, 2015, the FBI, the United States Department of Defense, The Innocence Project, and The National Association of Criminal Defense Lawyers published a report disqualifying hair follicle evidence as scientifically valid. According to the report, false statements were made in 96% of the reviewed cases in which experts relied on microscopic hair analysis to accuse a defendant.

Juries rely heavily on forensic science to make determinations of guilt; therefore, it is important that forensic science is accurate. There is no room for error. Inaccurate forensic science, or “junk science,” perpetuates wrongful convictions by leading juries to see guilt where there is no actual proof.

Cameron Todd Willingham 

Cameron Todd Willingham and his family (Photo from The New Yorker)

This was likely the case for Cameron Todd Willingham who is widely believed to be innocent despite being executed for triple homicide and arson.

In 1991 in Corsicana, Texas, 23-year-old Willingham’s home set fire early in the morning. Willingham, his one-year-old twins, Kameron and Karmen, and his two-year-old, Amber, were all inside. Willingham was observed doing everything he could to get inside to rescue his daughters, but he was unable to. Even as firefighters arrived, Willingham was attempting to get in and ultimately had to be restrained in handcuffs for his own safety. One firefighter told the New Yorker that it would have been crazy for anyone to go into the house based on the severity of the fire. 

The three children died from smoke inhalation. Willingham’s wife, 22-year-old Stacy Kuykendall, who had left the house early on the morning of the fire, worked at a bar, and Willingham was an unemployed auto mechanic. The small town, which was about 55 miles north of Waco, collected money to help the Willinghams pay for the funerals.

Douglas Fogg, the assistant fire chief in Corsicana, and Manuel Vasquez, a deputy fire marshall, investigated the fire. Their findings were that someone poured liquid accelerant in the children’s room, under each of their beds, and then down the hallway towards the front door. They called it a “fire barrier” and concluded that the accelerant was deliberately poured so that no one could escape. A lab found mineral spirits, a substance often used in lighter fluid, in one of the samples of burned materials from the house.

Just like that, the tragic fire became a triple homicide. The closest thing the prosecution had to a motive was that Willingham had Iron Maiden and Led Zeppelin posters in his house, which a family counselor who had never met Willingham testified indicated an interest in “satanic-type activities.” 

Willingham was convicted of murdering his three girls and sentenced to death. Before Willingham’s execution, his attorney enlisted Dr. Gerald Hurst, an acclaimed scientist and fire investigator, to review the initial investigation. He had little doubt that it was an accidental fire that was likely caused by a space heater. Vasquez recalled finding the space heater off, but Stacey was sure it was on. She remembered turning it down before she left because Amber was always putting things too close to it. Hurst told the Board of Pardons and Parole that there was no evidence of arson and that Willingham was about to be executed based on junk science. 

Despite receiving this report, the board unanimously voted to move forward with the execution. Willingham died by lethal injection in 2004. He requested that his parents not be present. Prior to his execution, Willingham asked them to never stop fighting to vindicate him. He also requested to be cremated and for them to sprinkle some of his ashes over his children’s graves. 

Later that year, the Chicago Tribune asked three fire experts to examine the original fire investigation, and they agreed with Hurst’s findings. Shortly after, the Innocence Project hired four top fire investigators to do the same thing. They concluded that every indicator of arson that Fogg and Vasquez relied on had been proven scientifically invalid. In 2005, Texas established a commission to investigate allegations of error in forensic science. Willingham’s case was the first to be reviewed. Fire scientist Craig Beyler concluded that there was no scientific basis for arson, that Fogg and Vasquez ignored evidence, relied on discredited folklore, and that their reasoning was more “characteristic of mystics or psychics.” Most notably, Beyler told the New Yorker that the investigation violated “not only the standards of today but even of the time period.”

Click here to listen to the Wrongful Conviction: Junk Science podcast which explores how pseudo-sciences put the wrong people behind bars.

Relevant MTIP Case:

Bernard Pease

Fred Lawrence and Paul Jenkins