Why are only certain parts of DNA used for fingerprinting?

In one line

The rule

DNA fingerprinting uses regions of DNA that show high variation between individuals, called variable number tandem repeats (VNTRs) or short tandem repeats (STRs).

These regions are chosen because they are different in almost everyone, except identical twins.

Let's understand it simply

Imagine you and your friends all have a notebook. Most pages are the same—like the rules, the index, and the title page. But there is one page where you all write your own secret code or doodle. If someone finds just that page, they can tell whose notebook it is, even if the rest looks the same.

DNA is like that notebook. Most of it is the same in all humans—like the pages with rules. But some small parts are different for each person. These are the parts scientists use for fingerprinting. They look for the 'secret code' pages, not the common ones.

How it works

1. Scientists collect a DNA sample (like from hair, blood, or saliva).
2. They look for special regions in the DNA called VNTRs or STRs (these are repeating patterns that vary a lot between people).
3. They use enzymes to cut the DNA at certain places, so only these regions are separated.
4. The pieces are sorted by size using a process called gel electrophoresis (like sorting cricket balls by size).
5. The pattern of these pieces creates a unique 'barcode' for each person—this is the DNA fingerprint.

The story behind it

DNA fingerprinting was developed by Sir Alec Jeffreys in 1984 in England. He was studying how genes are passed from parents to children. One day, he noticed that certain regions of DNA varied a lot between people, even in the same family. He realized these regions could be used to identify individuals. His first big case was to solve a crime and prove family relationships.

Where you see it in real life

• Crime scenes: Police match DNA from blood or hair at a crime scene to a suspect using these unique regions.
• Paternity tests: To check if a man is the father of a child, scientists compare these DNA regions.
• Disaster victim identification: In train or plane accidents, DNA fingerprinting helps identify bodies.
• Wildlife protection: To catch poachers, DNA from animal remains is matched to known animals.
• Missing person cases: Families can be reunited by matching DNA fingerprints.
• Sports doping tests: To check if athletes are using someone else’s urine, DNA fingerprinting is used.

Why it matters

• Accuracy: These regions make identification almost 100% certain, so innocent people are not wrongly blamed.
• Privacy: Only a tiny part of DNA is checked, so your whole genetic information is not revealed.
• Speed: Focusing on these regions makes the process faster and cheaper.
• Legal proof: DNA fingerprinting is accepted in courts for solving crimes and family disputes.

Common mistakes

• Thinking all DNA is different between people. In reality, about 99.9% is the same; only a small part is unique.
• Believing fingerprinting checks the whole DNA. It only checks certain regions that are known to vary.
• Confusing DNA fingerprinting with actual fingerprints on your fingers—they are different things.
• Assuming identical twins have different DNA fingerprints. They do not; their DNA is the same.

Remember it

DNA fingerprinting uses only the parts of DNA that are different for each person, like a personal barcode.