The genetics of supertasting

After almost 80 years of study, the genetics of taste sensitivity is still not completely understood. Just when one theory seems to account for all the facts, an exception to the rule pops up. Studying human beings may be a science, but it’s not always an exact one.

Most supertasters have a gene that appears to control their sensitivity to bitter tastes. I say “appears” because other genes also influence sensitivity to bitterness. So far geneticists have discovered 35 bitter receptor genes. According to a company that offers to test your genes, up to 20% of the variation in bitter taste sensitivity may be due to these other genes. A more academic source puts that number as high as 50%. Geneticists debate whether sensitivity to bitterness is even related to overall taste sensitivity. Experimental psychologists like Linda Bartoshuk continue to study supertasters, but many geneticists no longer use the term.

One question that interests both geneticists and those who study evolution is this: Why have nontasters survived? In the days before supermarkets, nontasters presumably couldn’t tell if what they were about to eat would be deadly. Over millions of years you’d expect natural selection to favor the dominant gene for bitter taste sensitivity. But roughly 25 percent of the population have two recessive genes for this trait and are classified as nontasters. Interestingly enough, it turns out that 25 percent of chimpanzees are also nontasters.

The evolution of taste

The original experiments with chimpanzees were done in the UK, just prior to World War II, by R. A. FIsher.

[T]here was some initial concern about whether it would be possible to determine from an animal’s reaction whether it was a taster or not; however, “the first animal they tested took them out of any embarrassment, because it looked at Fisher in his eyes, and spit at him.”

If you assume humans and chimpanzees inherited this particular tasting trait from a common ancestor, that would mean there have been both tasters and nontasters for 20 to 30 million years. That was the original hypothesis of the British researchers.

Recent advances in genetics lead scientists to a different conclusion. Now that it’s possible to do a molecular analysis of specific genes, the DNA of chimpanzees and humans can be compared. Slight differences in their bitter taste genes suggest the taster/nontaster distinction appeared independently in humans and chimps.

The figure below diagrams the two hypotheses. The splitting of the Y into two branches indicates the divergence of humans and chimpanzees into two different species. In the single origin theory, an ancestor of humans and chimpanzees had the dominant taster gene (T) and the recessive nontaster gene (t) before the two species diverged. In the separate origin theory, the recessive gene arose twice, once in each species, after the two species had diverged.

T is the dominant gene, t the recessive. Chimps and humans have both.

Image source: Genetics

Nontasters: We need you

Even if the taster/nontaster distinction appeared only a million years ago, rather than 20 to 30 million years, you’d still have to wonder why the trait survived. Supertasters learn not to eat bitter, poisonous berries after the first taste, not after they get a fatal stomach ache. The explanation could be that tasters and nontasters are sensitive to different sets of bitter tastes, and this combination of sensitivities gives an advantage to medium tasters.

In the diagram, T is the dominant half of the bitter sensitivity gene and t is the recessive half. We get these two halves from our parents. The genetics of bitter taste sensitivity is not actually as clear cut as this diagram suggests, but it’s close enough. If Mommy and Daddy are medium tasters (half dominant, half recessive), the laws of chance predict that 25% of their children will be supertasters (TT), 25% will be nontasters (tt), and 50% will be medium tasters (Tt).

In the days of hunting and gathering, if medium tasters were sensitive to a wider range of bitter tastes than supertasters, they would survive. And, as the diagram illustrates, as long as there are medium tasters, there will be nontasters.

So the question remains, is there a bitter taste that nontasters are sensitive to? We only know of one so far, but the answer is definitely yes.

The bignay tree is a tropical evergreen that grows in Africa, Australia, Southeast Asia, and southern Florida. The fruit of the bignay tree is used to make jellies, jams, wine, and pies. If you serve bignay pie to a supertaster, it tastes sweet. Nontasters find it unpleasantly bitter.

There may be other taste sensitivities that are unique to nontasters. Like many questions concerning the genetics of taste, we don’t yet know the answer. But what we’ve learned so far is actually pretty impressive. It all started with a simple accidental discovery in 1931: A chemical tasted bitter to some people and not to others. The pursuit of that curious fact has led to research and discoveries that continue to advance not only genetics and the theory of evolution, but our understanding of nutrition, the senses, and the brain.

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