The Genetics Of Olfaction

No two people smell exactly alike. That is, noses sense odors in individual ways. What one nose finds offensive, another may find pleasant, while another might not smell anything at all. Scientists have long known the way things smell to us is determined by our genes.

Now, two studies appearing in the journal Current Biology
have identified “the genetic differences that underpin the differences in smell sensitivity and perception in different individuals.” And while some of these differences merely help determine our culinary preferences, others appear to play a subconscious role in how we choose our sexual partners.

For the first study, 200 people were tested to determine their sensitivity to 10 different chemical compounds commonly found in foods. The researchers found four of the ten odors had a genetic association. These were malt, apple, blue cheese, and a floral scent associated with violets.

The research team, led by Sara Jaeger, Jeremy McRae, and Richard Newcomb of Plant and Food Research in New Zealand, used a genome-wide association study. Their first task was to identify which test subjects could smell each chemical compound and which could not. They then searched the subjects’ genomes for areas of DNA that differed between these people.

“We were surprised how many odors had genes associated with them. If this extends to other odors, then we might expect everyone to have their own unique set of smells that they are sensitive to,” explained McRae

“These smells are found in foods and drinks that people encounter every day, such as tomatoes and apples. This might mean that when people sit down to eat a meal, they each experience it in their own personalized way.”

They further found there is no regional differentiation. A person in one part of the world is just as likely to be able to smell a particular compound as a person in another part of the world. In addition, sensitivity to one compound does not predict the ability to smell another compound.

The genes that determine our ability to perceive certain odors all lie in or near the genes that encode olfactory receptors. These receptors occur on the surface of sensory nerve cells in the upper part of the nose. A particular smell is perceived when these receptor molecules bind with a chemical compound wafting through the nose, causing nerve cells to send an impulse to the brain and producing our sensation of smell.

For the violet smell, caused by a naturally occurring chemical compound known as β-ionone, the researchers were able to pinpoint the exact mutation in gene OR5A1 that determines whether the smell is perceived as floral, sour or pungent, and whether it is found to be pleasant.

These findings might have future marketing value. According to Richard Newcomb, “Knowing the compounds that people can sense in foods, as well as other products, will have an influence on the development of future products. Companies may wish to design foods that better target people based on their sensitivity, essentially developing foods and other products personalized for their taste and smell."

SEXY OR STINKY?

A separate study was conducted by Leslie Vosshall of the Rockefeller University Hospital. Humans have about 1,000 genes that influence smell, and around 400 of these are responsible for sensing a particular odor molecule.

Testing 391 human subjects, Vosshall studied olfactory responses to two closely related steroids, androstenone and androstadienone, which are found in male sweat. People generally have strong reactions to these steroids, finding them either sweet and florally or rank and noxious. The gene 0R7D4 determines the intensity of these odors as well as the perception of them being either pleasant or repulsive.

According to Vosshall’s report: “People who found the smell repulsive were more likely to have two functional copies of OR7D4; those who perceived it as a more mild smell tended to have one or two impaired copies of the gene.”

This study is part of the larger goal of understanding how genetic and neuronal factors influence behaviors.

A 2002 study published in Nature Genetics provided more insight into the effect of male pheromones on women. This study looked at the link between women’s preferences for the odors given off by men and a group of genes called the Major Histocompatibily Complex (MHC) which contribute to a persons’ immune response.

In this experiment, a group of 49 women were asked to smell 10 boxes. Some of the boxes held t-shirts worn by men with different MHC genes, and others contained familiar household odors such as bleach or cloves.

The t-shirts were worn by men who slept in them for two nights and avoided contact with other scents during that time, even to the point of avoiding other people. According to the report, “the women were then asked to rate each scent based on their familiarity, intensity, pleasantness and spiciness, as well as choose the one odor which they would choose if they had to smell it all the time.”

What the researchers found was the women did not choose the scents of men whose genes were similar to their own, nor did they choose those whose genes were too dissimilar. The women showed no preference for odors from men who had the same genes as their mothers, but did show a preference for odors from men who shared genes they inherited from their fathers.

Scientists believe there are two reasons for preferring a mate whose MHC genes are different than one’s own. One is that it would tend to create offspring with more genetic diversity and thus more robust immune systems. The other is it helps to avoid inbreeding.

Of course, when people choose their mates, there are a number of social factors that come into play as well. However, studies have shown married people tend to have different types of genes than their spouses.

So, the next time you like the way a person smells, keep in mind it may mean you have complementary genes.