The variation that can be applied in crime detection is

Fingerprints are the variation most applicable to crime detection. Every individual (including identical twins) has a unique set of ridge patterns on their fingertips. These patterns - classified as loops, whorls, and arches - are formed during foetal development and remain unchanged throughout life. This uniqueness and permanence make fingerprints an extremely reliable tool for identifying individuals.

In forensic science, fingerprints left at a crime scene (latent prints) can be lifted, analysed, and matched against databases of known prints to identify suspects or confirm the presence of a particular person. This technique, known as dactyloscopy, has been used in criminal investigations worldwide for over a century.

The other variations listed are not suitable for crime detection:

• Tongue rolling is a genetic trait (the ability to roll the tongue into a tube shape), but it is shared by a large proportion of the population and cannot distinguish one individual from another.
• Height is a continuous variation influenced by both genetics and environment. Many people share similar heights, making it useless as a unique identifier.
• Colour (skin, eye, or hair colour) is also shared among large groups and provides no individual-level identification.

Exam tip: Fingerprints are an example of discontinuous variation - each person's pattern is qualitatively distinct, not just a point on a continuous scale. This uniqueness is what makes them forensically useful.