Caught Red Handed by DNA Profiling

Joanna Cheng Thomas Jefferson High School for Science and Technology

Deoxyribonucleic acid, or DNA, is the very substance that defines each of our appearances and personalities. This crucial molecule has applications in fields far and wide, including the courtroom. The use of DNA in forensic science has expanded rapidly over the past few years and has proved itself to be indispensable evidence in difficult cases, but how exactly can a sequence of four letters, A, C, G, and T, tip the balance between exoneration and condemnation?

That microscopic sequence of four letters contains all of our genetic information deep inside our cells. Some parts of DNA are coding regions, which means that they control the creation of proteins, but a large percentage of our genome is non-coding. These non-coding regions of our DNA are partially made up of repetitive sequences, one type of which is called short tandem repeats, or STRs [2]. Even though more than 99.9% of our DNA is identical to other people’s DNA, the number of repeats in these STR sequences can vary among people, giving that 0.1% of individuality and identity to each person’s DNA [4].

A DNA profile consists of a set of twenty STRs mutually decided upon by scientists. Each person’s profile records the number of repeats in each STR, and these profiles can be compared to one created from a sample of biological material to identify who that sample belongs to [4]. Each STR can have up to 30 different variations among the human population, and considering that a profile documents many STRs, the chance that two people’s profiles match is astonishingly small [6]. To lend even more specificity and use to DNA profiles, STRs are inherited from our parents, giving us two copies, or alleles, of each one, often with different numbers of repeats [1]. It is essential that the chance of two profiles being the same is minuscule to allow confidence in DNA evidence, especially in grave situations such as criminal cases [6].

The process required to prepare and analyze DNA from biological material is no easy task. First, forensic scientists must collect a sample of biological material that contains DNA, such as blood or saliva. Then, to extract the DNA from the sample, scientists use chemicals to lyse, or break down the cell membrane and separate the DNA from other parts of the cell [3]. Next, the polymerase chain reaction (PCR) amplifies, or copies, the DNA to allow for facilitated analysis. The first step of PCR is called melt, in which high temperatures separate the two strands of DNA. Primers, short DNA sequences that can bind to the start and end of the target DNA section, then attach to the separated DNA strands in the second step, the anneal step. Finally, an enzyme called DNA polymerase carries out the last step, called extend, by building the DNA sequence using the primers as starting points. This cycle is repeated up to 28 times to generate up to a billion copies of a segment of target DNA [5]. The last step is to determine the number of repeats of each STR. Gel electrophoresis can separate DNA fragments by size and quantity. Scientists inject the DNA samples into one end of a gel and run an electric current through the gel, which causes the DNA fragments to move to the other end. Since larger fragments move through the gel more slowly than smaller fragments, by comparing the distance traveled by different segments to a reference called an allelic ladder, scientists can determine the number of repeats in that STR [1].

STRs are perfect for use in DNA profiling because they are relatively short and are more likely to be preserved in cases of extremely damaged DNA samples [6]. However, forensic scientists have discovered other genetic marker systems, each with their own benefits and drawbacks. Single nucleotide polymorphisms, or SNPs, are single nucleotide variations among the human population. There are millions of SNPs in the genome with low mutation rates. In addition, SNPs are only one nucleotide long and are therefore more easily obtained from damaged DNA, indicating possible applications in DNA profiling [2]. The biggest disadvantage of SNPs is that each one provides far less information than a STR, so a detailed profile must contain many more SNPs [3].

The following two DNA profiling techniques are more often used to identify lineage from parent to child. Y-chromosome testing is advantageous in cases involving a male perpetrator and a female victim. Since only males have Y-chromosomes, any Y-chromosome fragments in a sample must be from the perpetrator [3]. Another application is the tracing of lineage from father to son. However, since fathers pass Y-chromosomes to sons with little recombination, there is much less variety among Y-chromosomes, so they do not provide very unique profiles, but they prove to be useful for paternity tests [2]. On the other hand, mitochondrial DNA (mtDNA) can trace lineages from mother to children, since children inherit mitochondria solely from the mother. mtDNA analysis is useful in cases when regular nuclear DNA is scarce or damaged, since each cell has a greater number of molecules of mitochondrial DNA than regular DNA. Nevertheless, PCR can be difficult to perform with mtDNA. It also has high mutation rates, which make it more difficult to trace lineages. Because all current databases use STRs as the standard, it is unlikely that new profiling techniques will overtake STRs. However, they still hold a place in the process of paternity and maternity tests [3].

DNA profile databases all around the world have greatly changed the workings of forensics by connecting serial cases, freeing wrongly accused individuals, and much more. The United States stores DNA profiles in the Combined DNA Index System (CODIS), which now contains the profiles of over 16 million convicted offenders [1]. There are three possible types of searches in CODIS: high stringency, medium stringency, and low stringency. In a high stringency search, all the available alleles must match in the two profiles. A moderate stringency search requires all available alleles to match, but the two profiles can contain a different number of alleles due to DNA mixture or degradation. A low stringency match occurs when at least one allele matches and is useful in searches for familial relationships [3].

There are, however, many concerns over DNA databases, one of the foremost being privacy. These databases store sensitive genetic information that could threaten people’s sense of security if mishandled [7]. Governments have passed many laws to uphold privacy concerning genetic data. Firstly, the twenty main STRs come from non-coding locations in the genome that cannot give any information about genetic diseases or predispositions. Secondly, national databases do not store any identifying information such as names and simply store the local source of the DNA profile. That way, one can only obtain personal information by submitting a request to the local crime laboratory or police station [3].

Critics of DNA profiling have also brought attention to accuracy issues within the process. PCR’s sensitivity has allowed scientists to glean information from minuscule amounts of DNA, but there is a downside: these methods are now sensitive enough to detect background DNA. For example, someone may have touched the crime scene before the criminal did, and their DNA may now be mixed into the sample. Forensic scientists are not perfect either; human error in the complex process may result in contamination or incorrect results. Furthermore, during the PCR process, amplification may drop or add repeats, a phenomenon known as stutter peaks [1]. Additionally, the precision of DNA profiling is greatly diminished if an intact, uncontaminated, and sizable DNA sample is unobtainable. If a DNA sample is too small, forensic scientists use methods such as increasing the amount of PCR cycles to better amplify the DNA sample [3], but these enhanced techniques may lead to some unwanted consequences such as allele drop-in or drop-out. Allele drop-in occurs when additional alleles are observed due to contamination, while allele drop-out occurs when an allele fails to amplify [2, 7]. All of these phenomena can lead to false conclusions and incorrect analyses, potentially causing an unjust decision to be made in the courtroom.

Despite these issues with accuracy, scientists have used DNA profiling in many other situations besides forensics, such as in the identification of disaster victims and unidentified remains. Family members can give DNA samples for comparison against databases with profiles of unidentified remains. This technique has allowed news of victims of war, natural disasters, airplane crashes, and terrorist attacks to reach home, giving closure and peace to living relatives [3].  

The future of DNA profiling holds much to look forward to. Companies are working towards fast and efficient DNA profiling, in which police or other non-scientifically trained personnel can take cheek swabs and receive a profile in less than 20 minutes. Scientists are even working on using DNA sequences to build a picture of the suspect by analyzing genes that contribute to hair, eye, and skin color. Certain molecular markers on DNA also hold promise in forensics by allowing scientists to predict a suspect’s age [1].

With new technology always comes new concerns. These privacy and accuracy issues must be considered every time a scientist uses DNA profiling to testify at the stand. However, it is safe to say that the advent of DNA usage in the courtroom has closed countless cases and revolutionized forensics.

References

[1] Arnaud, C. H. (2017, September 18). Thirty years of DNA forensics: How DNA has revolutionized criminal investigations. Chemical & Engineering News, 95(37), 16-20. Retrieved from https://cen.acs.org/articles/95/i37/Thirty-years-DNA-forensics-DNA.html 

[2] Barh, D., & Azevedo, V. (Eds.). (2017). Omics technologies and bio-engineering. https://doi.org/10.1016/C2015-0-01634-3 

[3] Butler, J. M. (2012). Advanced topics in forensic DNA typing: Methodology. https://doi.org/10.1016/C2011-0-04189-3 

[4] DNA profiling. (2005, December 1). Retrieved June 1, 2020, from https://www.sciencelearn.org.nz/resources/1980-dna-profiling 

[5] Godbey, W. T. (2015). An introduction to biotechnology. https://doi.org/10.1016/C2013-0-18161-5 

[6] Pyrek, K. M. (2007). Forensic science under siege: The challenges of forensic laboratories and the medico-legal investigation system. https://doi.org/10.1016/B978-0-12-370861-8.X5000-1 

[7] Trent, R. J. (2012). Molecular medicine: Genomics to personalized healthcare (4th ed.). https://doi.org/10.1016/C2009-0-61768-2