• Researchers conducted a smell test on over 9,000 participants 
  • Some of them couldn't detect the fish scent or had a less intense experience 
  • Researchers found for these people the fish's off-putting odor might smell even like caramel

A new study has revealed a genetic mutation that makes people who carry it be less susceptible to the smell of fish. For some of these people, fish's off-putting odor might even smell like caramel or roses.

Humans perceive smells using olfactory receptors, but of the 855 olfactory genes, about half are actually not functional. The reasons for this and why people have a "highly personalized" sense of smell remain a mystery.

To shed light on the matter, the researchers of a new study conducted a smell test wherein they presented 9,122 participants in Iceland with various smells including fish, cinnamon and liquorice, then asking them to name it and rate the pleasantness and intensity.

Overall, fish scent was rated as the least pleasant, but a small number of the participants had a somewhat different perception of the fishy smell.

The researchers found people with a mutation in a receptor gene called trace anime-associated receptor 5 (TAAR5) perceived the fish odor with trimethylamine (TMA) as a main ingredient as less intense. For instance, when presented with the smell of fish, those with the variant described it as having the smell of "caramel" or "rose" instead of other fishy odors such as "fermented skate" or "shark." Some could not even recognize the scent at all.

On the other hand, those with an intact version of TAAR5 easily recognized the fishy scent.

"Carriers of the variant find the fish odor less intense, less unpleasant, and are less likely to name it accurately," study first author Rosa Gisladottir of deCODE Genetics in Iceland said in a Cell Press news release.

Exactly how this variant affects the odor perception of people with the mutation is still unclear.

TMA is also found in animal odors and human body secretions such as urine, blood and sweat.

Pictured: Representative image of a crate of fish at a market. Pixabay

Additionally, the researchers also identified other variants that affected people's ability to perceive the scent of liquorice and cinnamon. In this case, the participants found the scents to be more intense but also more pleasant.

These particular variants are distributed differently across the globe -- 57% in East Asia and 11% in Europe.

"Spices containing trans-anethole, the main ingredient in the licorice odor, are widely used in Asian cuisine and traditional medicine," the researchers wrote. "It could be speculated that variants associated with the perception of trans-anethole licorice odor conferred some advantage in East Asia, leading to the large frequency differences between populations."

The study showed that even if humans do have reduced olfactory genes, the differences in variants make the sense of smell quite diverse.

"Altogether, our results provide a unique window into the effects of sequence diversity on human olfaction," the researchers wrote. "An individual's personalized OR repertoire gives rise to myriad differences in perception and behavior, including olfactory language, which we are only beginning to understand."

The study is published in the journal Current Biology.