Uncovering Your DNA


Marissa Kramer '23, Guest Writer

Clothing, silverware, bedding, toothbrushes, hairbrushes, and much more have one thing in common: DNA. Our DNA is left everywhere through skin cells, blood, hair, and other things, but we never realize what is left behind. There is a select group of people called forensic scientists that can identify who we are based on the single strand of hair left on a chair. For those of you that don’t binge-watch Law & Order, forensic science is the process of examining and analyzing evidence from a crime scene and elsewhere to assist in the investigation and prosecution of criminals. In 1987, forensic DNA analysis was seen for the first time in a courtroom. However, it was first known as DNA fingerprinting, whereas now it is called DNA profiling or DNA testing.

Almost all cells in our human body have DNA containing three billion DNA base pairs. Any two people share an average of 99.9% of their DNA, which means that only 0.1% is unique to oneself. The o.1% of the three billion base pairs equals three million base pairs. 

When dealing with DNA fingerprinting, scientists look at the different patterns. This is called a Short Tandem Repeat, or STR. STR consists of a string of repeating DNA nucleotides that are usually two to five bases long. One thing to remember is that everyone has the same STR at the same place in the genome. However, the number of repeats that occurs is different between people. The process of DNA fingerprinting starts by collecting samples from the crime scene. The part that contains each STR that is being analyzed is then copied and separated based on their size. The model is then compared to the sample taken from the suspect. If the sample does not match, the person is eliminated from the investigation as a potential suspect. 

In a recent study done in 2017 by the National Institute of Standards and Technology (NIST), scientists tested the reliability of forensic methods for analyzing evidence. The study focused on DNA mixtures involving three or more people as well as Touch DNA, which is the use of very small quantities of DNA such as skin cells. When evidence contains DNA from only one or two individuals, the DNA profiles are considered more reliable. If evidence contains DNA from three or more individuals, it can be difficult to separate each individual’s DNA. During investigations, police now swab door handles and other surfaces for Touch DNA at a crime scene. NIST Fellow and forensic DNA expert John Butler states, “Some labs won’t do anything with that kind of evidence, other labs will go too far in trying to interpret it.” (Butler, 2017)

The study focused on the reliability of forensic methods when dealing with different types of DNA evidence, for example, the difference between two-people mixtures vs four-people mixtures and taking into consideration the amount of DNA. Labs, courts, and other institutions can then use the information from this study to determine which method is best suited for the case. Butler says, “The goal is not to undermine these methods, but to determine their bounds or reliability so they can be used appropriately” (Butler 2017).