Genetics: quirk means up to 2.2 per cent of people can’t smell fish and may confuse it with roses

‘A rose by any other name would smell as sweet,’ declaims the titular tragic hero in William Shakespeare’s ‘Romeo and Juliet’. But what if that rose were really a fish?

For up to 2.2 per cent of people, the stench of fish is indistinguishable from sweet roses, caramel or nothing at all — with a genetic quirk responsible, a study found.

Researchers from Iceland analysed the genomes of more than 9,000 people — and compared this with their performance in various smell tests. 

The team also revealed that many people smell cinnamon and liquorice differently.

In humans, odours are detected using so-called olfactory receptors, which are encoded in our DNA by 855 olfactory genes — only around 400 of which function.

The loss of so many olfactory genes remains a mystery — as does exactly how genetic variations between people lead to different senses of smell. 

‘We discovered sequence variants that influence how we perceive and describe fish, liquorice, and cinnamon odours,’ said paper author and geneticist Rosa Gisladottir of deCODE Genetics in Reykjavik, Iceland.‘

Since our sense of smell is very important for the perception of flavour, these variants likely influence whether we like food containing these odours.’ 

In their study, Professor Gisladottir and colleagues analysed the DNA of 9,122 Icelanders, with a focus on the genomes involved in our sense of smell.

Each participant was also asked to take a series of smell tests, in which they had to sniff the odour released from pen-like scent devices — and subsequently identify the aroma, rate its intensity and rank it based on how pleasant they found the smell.

Scents used in the test, the researchers explained, included those of fish, banana, cinnamon, lemon, liquorice and peppermint.

The team identified three specific variations between the subjects’ genomes — the existence of which was confirmed in an separate study of 2,204 more Icelanders.

The first is an olfactory receptor gene called trace amine-associated receptor 5 — or ‘TAAR5’ for short — which changes the way that its holders perceive trimethylamine, a compound found in both rotten and fermented fish.

From the smell tests, the researchers found that people with a certain variant of this gene typically either cannot smell fishy odours — or alternatively described the smell with positive or neutral descriptors as like ‘roses’, ‘caramel’ or ‘potatoes’.

‘Carriers of the variant find the fish odor less intense, less unpleasant, and are less likely to name it accurately,’ Professor Gisladottir said. 

‘There is a lot of animal research on TAAR5 in relation to its role in hard-wired aversive responses to trimethylamine.’

‘Our findings extend the implications of this research to human odor perception and behaviour,’ added Professor Gisladottir. 

The researcher’s other two discoveries — which impact the ability of some individuals to pick up on the odours of cinnamon and liquorice — were found in more common olfactory gene variants.

‘We discovered a common variant in a cluster of olfactory receptors which is associated with increased sensitivity to trans-anethole,’ Professor Gisladottir.

This, she explained, is ‘found in black liquorice products but also in spices and plants such as anise seed, star anise and fennel.’

‘Carriers of the variant find the liquorice odor more intense, more pleasant and can name it more accurately,’ she continued.

‘Interestingly, the variant is much more common in East Asia than in Europe.’

The cinnamon variant, meanwhile, makes some people have a heightened sense for the major ingredient in both Chinese and Ceylon cinnamon, trans-cinnamaldehyde.

The researchers noted that the frequency of the variants differs between populations. TAAR5 appears in 2.2 per cent of Icelanders and 1.7 per cent of Swedes, but only in 0.8 and 0.2 per cent, respectively, of Southern Europeans and Africans.

‘Coupled with evidence for geographical differences in allele frequencies, this raises the possibility that the […] diversity found in human olfactory receptor genes that affects our sense of smell is still being honed by natural selection,’ the team wrote. 

With their initial study complete, Professor Gisladottir and colleagues are continuing to explore how odour perception varies between people — alongside using similar experimental tests to investigate the loss of smell associated with COVID-19.

The full findings of the study were published in the journal Current Biology.