Whose Blood Is It? How Forensic Scientists Read DNA at a Crime Scene
In 1986, British scientist Alec Jeffreys used a new technique called DNA fingerprinting to solve a double murder case, and in doing so also proved that an innocent man who had already confessed under pressure was not the killer. The real culprit, Colin Pitchfork, became the first person in the world convicted using DNA evidence. Forensic biology is the science of reading the body's own genetic code to answer one question: who was really there?
What You'll Learn
- What DNA is and why every person's is unique (except identical twins) - How forensic scientists extract usable DNA from blood, skin cells, or saliva - What a DNA profile looks like and how two profiles get compared - Why DNA evidence can both convict and exonerate people
Finding DNA Where You Would Never Look
DNA is not just in blood. It is in skin cells left on a doorknob, saliva on a coffee cup, hair follicles, and sweat on a steering wheel. Forensic biologists swab these surfaces, then use a process called PCR, or polymerase chain reaction, to copy the tiny amount of DNA millions of times, because a single skin cell does not contain enough DNA to test directly. PCR was invented by Kary Mullis in 1983 and won him the Nobel Prize in Chemistry in 1993; it is now one of the most important tools in all of biology, not just forensics.
Reading a DNA Profile
A DNA profile does not read your entire genetic code like a movie script. Instead, it looks at 13 to 20 specific short spots in your DNA called STRs (short tandem repeats), where the number of repeats varies a lot from person to person. The FBI's CODIS database compares 20 of these specific spots. The chance that two unrelated people share the same profile at all 20 spots is smaller than 1 in 1 trillion, which is why a full match is treated as extremely strong evidence, while a partial or degraded sample is treated far more cautiously.
When DNA Sets People Free
DNA evidence does not only convict; it also exonerates. The Innocence Project, founded in 1992, has used DNA testing to help free more than 240 wrongfully convicted people in the United States, some of whom had spent over 20 years in prison for crimes they did not commit. This is why forensic biologists are trained to follow strict chain-of-custody rules: every person who touches a sample must log it, so a defense team can later verify the evidence was never contaminated or swapped.
DNA at a scene shows presence, not guilt. Your DNA could be on a doorknob because you live there, not because you committed a crime. Forensic scientists testify about probability and match likelihood; juries must weigh DNA evidence alongside motive, opportunity, and other proof.
Match each forensic biology term to its meaning.
Terms
Definitions
Drag terms onto their definitions, or click a term then click a definition to match.
Why do forensic scientists use PCR before testing a DNA sample found at a crime scene?
What is the most important lesson from the Innocence Project's 240+ exonerations using DNA?
Build a Mock Chain-of-Custody Log
1) Imagine a swab of saliva was collected from a water bottle at a mock crime scene. 2) Create a chain-of-custody log with columns: Name, Role (collector, lab tech, analyst), Date/Time, Action Taken (collected, transported, tested, stored). 3) Fill in at least 5 rows showing the sample moving from the scene to a lab result. 4) Deliverable: your completed log table plus 2-3 sentences explaining what would happen to the case if one row in your log were missing.
Want to keep learning?
Sign up for free to access the full curriculum — all subjects, all ages.
Start Learning Free