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

The stench of fish is indistinguishable from sweet roses, caramel or nothing at all for up to 2.2 per cent of people — 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. 

 For up to 2.2 per cent of the population, the stench of fish is indistinguishable from fragrant roses or sweet caramel — and a genetic quirk is responsible, a study found (stock image)

‘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. 

Each participant was asked to take a series of smell tests, in which they had to sniff the odour released from pen-like scent devices (pictured) — and subsequently identify the aroma, rate its intensity and rank it based on how pleasant they found the smell. Scents used in the test included those of fish, banana, cinnamon, lemon, liquorice and peppermint

Each participant was asked to take a series of smell tests, in which they had to sniff the odour released from pen-like scent devices (pictured) — and subsequently identify the aroma, rate its intensity and rank it based on how pleasant they found the smell. Scents used in the test included those of fish, banana, cinnamon, lemon, liquorice and peppermint

Genetics: quirk means up to 2.2 per cent of people can't smell fish and may confuse it with roses 1

Genetics: quirk means up to 2.2 per cent of people can't smell fish and may confuse it with roses 3

From the smell tests, the researchers found that people with a certain variant of the TAAR5 gene typically either cannot smell fishy odours — or alternatively described the smell with positive or neutral descriptors as like ‘roses’ (left), ‘caramel’ (right) or ‘potatoes’

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.

'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' Pictured: liquorice allsorts

‘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’ Pictured: liquorice allsorts

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.

HOW DOES THE HUMAN OLFACTORY SYSTEM WORK?

The human olfactory system (artist's impression)

The human olfactory system (artist’s impression)

Smelling is a complex process.

The olfactory systems detects molecules in the air. 

Breathed into the nasal cavity, odorous molecules come into contact with olfactory epithelial tissues at the top of the nose.

Each molecule stimulates multiple chemical receptor cells.

The olfactory nerve passes the information from the receptor cells into the brain for processing.

The information transmitted includes measures of the odour intensity and quality. 

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