Banner by Dhara Patel

Beyond a Shadow of a Doubt: DNA Sequencing and Criminal Justice

By Sarah Lindley

Thirty-eight years. That is how long Malcolm Alexander spent in prison for a sexual assault he did not commit before he was exonerated in 2018. He was initially convicted based on improper victim identification procedures and his lawyer prepared his case so inadequately that he was later disbarred. The exoneration came down to a major piece of evidence: DNA found at the scene that matched neither Alexander nor the victim, but a third individual. The fields of forensic science and criminal justice have been irreversibly altered by the development of DNA sequencing techniques. One remarkable aspect of this technology has been its assistance in successfully appealing wrongful criminal convictions. According to the Innocence Project — a nonprofit dedicated to this very mission — since 1989, 375 wrongly convicted individuals in the United States have been exonerated after newly applied DNA evidence proved they were not guilty. Twenty-nine percent of those individuals were convicted after giving a false confession, and 60%, like Alexander, have been African-American, even though African-Americans only make up 13.6% of the U.S. population. Now, 165 of the people who actually committed the crimes in question have been identified. Clearly, DNA technology has made an astounding difference in helping individuals get the justice they deserve after losing precious years of their life to serving wrongful sentences — but how does it actually work? 

 DNA sequencing in forensic science revolves around short tandem repeats (STRs), which are small, non-coding sequences of DNA that repeat a different number of times in different people. For instance, the sequence “GATA” might repeat eight times in a row at one spot in my DNA, while in yours, it may repeat 10 times. The number of times the sequence repeats is considered an allele — a version of a gene — just like there are alleles for a widow’s peak or color blindness. Individuals may also have two different alleles for each STR, since one is inherited from each parent; someone might have received an allele of eight repeats from their mother and an allele of nine repeats from their father. These STRs are found every 30,000 or so base pairs or “letters” in our DNA, so considering that our genome runs billions of base pairs long, they are rather common.                                                             

When someone’s DNA is found at a crime scene, it is not their entire genome that gets sequenced, but rather a standardized set of these STRs, which have been pre-selected as markers in the Combined DNA Index System (CODIS) database. The CODIS database initially had 13 markers, but seven more were added in 2017. Each of these STRs has a handful of different alleles that are common in the population, so considering all these locations, the odds of any two individuals having the exact same STRs is incredibly small; between all 21, the chances of even siblings having identical combinations is less than one in a trillion, and much less for unrelated people. 

STR analysis provides much lower odds of misidentification compared to other types of evidence that are often used in court and are significantly more unreliable. Take eyewitness identification, for instance. People are generally worse at identifying suspects that are not within their own racial group. Misidentification of suspects by white people is a contributing factor to why Black people in the U.S. are 3.5 times more likely to be wrongly convicted of sexual assault (like Alexander), seven times more likely to be wrongly convicted of murder, and 12 times more likely to be wrongly convicted of drug crimes. Unfortunately, this hardly scratches the surface of the bias that Black individuals face in the justice system. They are more likely to be stopped and searched at traffic stops, have force used against them by the police, be charged as an adult while still a juvenile, and live in impoverished neighborhoods with higher crime rates that are susceptible to more aggressive policing systems and policies. 

STR evidence is not always perfect, but it gives justice a fighting chance. DNA does not care about race, and it is much harder to embellish or falsify than eyewitness identification or false confessions. However, it is critical to examine what role DNA sequencing technology can and should play in the future of forensic science and criminal justice. Now that we have this technology, we are unlikely to go backwards and abandon it, so the most realistic option is to adhere to guidelines to ensure everyone involved is accountable and make sure every step of this process, from DNA sample collection to analysis of the results, is done ethically, responsibly, and appropriately. In the U.S., individuals of all racial backgrounds are presumed innocent and deserve the peace of mind in knowing that they will not be convicted for a crime unless they are proven guilty beyond a shadow of a doubt.