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November 21, 2018

The diverse tastes of the season

Filed under: Food,Genetics,science,thanksgiving — Razib Khan @ 1:31 pm

The holiday season is upon us. This means food, family, and fun. And when it comes to food and drink it often means excess. People gain weight during the holidays, and that’s a function of our calorie budget. There are some surpluses you don’t want.

But the process all starts with the senses. The visual allure of bright sweets and the enticing golden richness of meats. The tactile textures as we munch on foods with their own complex physical “form factor.” And of course there is smell and taste, two senses which are intimately connected, as anyone who has a severe cold can tell you.

For decades, one particular element of taste has been associated with the illustration of genetics for high school students: the strong bitter that some people experience when tasting paper soaked in phenylthiocarbamide (PTC). About ~75% of the population experiences a strong reaction when they put this paper to their tongue, and ~25% do not.

Recessive inheritance of the trait defined by shaded individuals

People who are “non-tasters” for bitter for PTC paper are recessive to those who are “tasters.” For genes, humans have two copies, and to be a non-taster individuals have to have two copies of the non-taster variant. For the ability to taste, you need only one copy.

The implication of the frequencies of the trait above and the inheritance pattern is that the underlying frequency of the genetic variants was similar. About half the gene copies in the population were non-taster, and half taster.

This was the theory. But until modern genomics, this could only be inferred. But today we know the gene and the marker within that gene that is responsible for this classical Mendelian trait. It is TAS2R38.

Here is a plot of three different genetic variations from a recent paper:

PAV = “taster” variant

One thing that is immediately evident from this map is that the ability to taste PTC is widely distributed, as is the ability to not taste PTC. There are some characteristics, such as light hair, which is found in only a few populations. In contrast, dark hair is found in most populations. What you see with PTC tasting is that variation is found across all populations.

What researchers have found is that these genetic variations are ancient, going back ~1,000,000 years, well before the emergence of modern humanity. The maintenance of this variation so long tells evolutionary biologists that both variants are useful in some fashion, and diversity is maintained within the human species.

Brussel Sprouts

It turns out that variation on TAS2R38 correlates with variation in bitter taste more generally, and that one’s sensitivity to bitter predicts phenomenon such as how much alcohol people drink, or how many vegetables children eat. What is happening is that people who are non-taster for PTC have reduced bitter sensitivity overall, and also have a lower aversion to bitter food and drink.

Because of the high genetic variation on this gene, there is going to be differences within families in terms of perception. Some research has suggested that mothers who are non-tasters who have children who are tasters report more conflict around food and that their children are particularly “picky.”

And so goes bitter, so with salt, sweet, sour, and even umami. All of these tastes that we take for granted have a biological basis that is genetically mediated. They are functionally important. In our evolutionary history bitters were often unpalatable or even toxic. This is primal, going back to the roots of the tetrapod lineages, as plants and animals have long engaged in an evolutionary arms race. Sour keys us into the acidity of foods around us. Salt is an important nutrient which was often in deficit in the ancestral environment, as were very sweet foods. Finally, umami signals that the foods we eat are rich in protein.

Lutefisk

But as omnivores, our environments are protean, as our species migrated across the face of the earth. Over the past ten years, as the complex genetic molecular genetic underpinning of variation in taste have been uncovered, a theme of diversity has been reiterated. The genes underlying variation in taste also impact other traits, and one can conceive of a model where human populations expand and must face dynamic trade-offs in particular characteristics. It’s not just about taste. One can imagine scenarios such that populations where a mix of individuals differ in taste perception are more fit than those populations which are genetically homogeneous.

However that diversity came about, it’s a fact of our life. On a social and cultural level, it results in the wide array of foods and cuisines that we as humans can enjoy. Our palette’s flexibility means that people from different cultures can and do enjoy each other’s cuisine.

The diversity even manifests within families. So this Thanksgiving while you’re observing someone wolfing down something that you’d never think of putting in your mouth, remember that we’re all different, and that’s because of our evolutionary history.

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The diverse tastes of the season was originally published in Insitome on Medium, where people are continuing the conversation by highlighting and responding to this story.

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