The geography of taste

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The History and Geography of Human Genes

"The History and Geography of Human Genes" in 1994 was a monumental compilation and analysis of the human genes (or more exactly, the traits conferred by these genes) that were known at the time. In this book L-L Cavalli-Sforza and his colleagues presented detailed genetic arguments for population identities and movements, based on the distributions of the known genes in populations throughout the world.

One major feature of the analyses struck me at the time: the distribution of PTC tasting, which is shown in the map at left. As can be seen, with the exception of the aboriginal populations of Australia, the populations with the lowest prevalence of taste sensitivity to PTC are located in Europe, the Middle East, and India. By the authors' calculations, PTC non-tasting was associated with populations that had entered Europe from the Middle East after the last Ice Age. Presumably these were also the people who entered Western India in several waves over time. VU

The genes for bitter tastes

Our ability to experience bitter tastes depends on the number and types of receptors for bitter chemicals that our taste buds have. Receptors are special chemical molecules that sit in the membrane around taste cells, waiting for chemicals in our foods to bind to them. When a bitter chemical lands on its receptor, it joins with the receptor in a lock and key fashion. By binding in this way, the receptor changes its shape, and the change in shaoe lets the inside of the cell know that the bitter chemical is in the food. The cell then sends this message to the brain through the pathways we have described under From tongue and nose to brain.

Creation of the receptors depends on genes in our DNA. There is a family of genes called the T2R genes that provides the genetic code that our taste cells need to manufacture an array of different taste receptors. Each receptor type appears to bind a different bitter compound. Over time, these receptor genes have mutated so that people may make some receptors that do not bind to their appropriate bitter compounds anymore, or that may bind to a different compound, or people may fail to make certain receptors altogether.

The geography of taste

The map above is derived from data presented by LL Cavalli-Sforza and his colleagues in The History and Geography of Human Genes, and shows the prevalence of the ability to taste a bitter chemical, phenylthiocarbamide (PTC), The darker the area in this map the higher the percentage of the native population of the area who can taste PTC. As you can see, the percentage varies considerably, being over 85% in the darkest areas, and as low as 5% in the lightest.

PTC tasting

Phenylthiocarbamide is a more toxic chemical cousin of PROP, that has been used extensively to explore individual differences in taste sensitivity. The gene for the capacity to taste PTC was discovered by Kim and colleagues and reported in 2003. They described variants of the gene that conferred the ability to taste PTC which made subjects unable to taste the chemical. Kim and colleagues have gone on to show that different geographic patterns appear to exist for mutations of the different bitter receptor genes, and that there is an unusually high degree of variability in these genes from population to population (Kim, U.; Wooding, S.; Ricci, D.; Jorde, L. B.; Drayna, D.: Worldwide haplotype diversity and coding sequence variation at human bitter taste receptor loci. Human Mutation 26: 199-204, 2005). These authors suggest that we may have adapted to recognize just the specific chemicals in our individual environments.

A complex pattern

This discussion of bitter receptor genes just skims the surface of a very complex pattern of taste abilties, that varies from person to person. When we discuss mildly, moderately, and highly sensitive tasters, we are probably grouping together people who differ in ways we have not tested. Therefore our findings should be considered in light of the specific tastant we are using for our tests. We also recognize that, irrespective of tastant, the more taste buds you have, the stronger the taste of compounds that you are genetically programmed to taste, so a division of people into the three types is a useful tactic for discovering how taste sensitivity relates to other activities of the brain and mind.

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