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 shape 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 for the manufacture of an array of different taste receptors. Each bitter receptor type appears to bind to a particular set of bitter compounds. Over time, these receptor genes have mutated so that people may make some receptors that do not bind to the original bitter compounds anymore, or that may bind to a different compound, or people may fail to make certain receptors altogether.
Kim and colleagues have shown 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).
The geography of PTC tasting
Phenylthiocarbamide is a chemical that has been used extensively to explore individual differences in taste sensitivity. Kim and colleagues discovered the gene, TAS2R38, for the capacity to taste PTC and found variants of the gene that made people unable to taste the chemical.
As Cavalli-Sforza and colleagues had suggested (see sidebar), there are significant differences in the prevalence of sensitivity to PTC between Africans and non-Africans. Recently, a thorough study of the mutations in the TAS2R38 gene both in and out of Africa showed that these mutations have a very ancient origin, appearing before the development of Homo sapiens. The major variations, which lead to either the ability or the inability to taste PTC, have been maintained in hominid populations ever since, suggesting that there are advantages to having both tasters and non-tasters of PTC in human groups (see sidebar for these mutations in Neanderthals!).
I derived the pie charts here from a paper by Tishkoff and her colleagues in which they describe the frequency of mutations in the TAS2R38 gene in Africans and non-Africans. Blue shows the frequency of the gene variant that confers the ability to taste PTC, red shows the frequency of the major non-taster variant, and grey shows the frequency of variants that confer intermediate taste sensitivity. The top pie shows the results in Africans, and the bottom pie the results in non-Africans. While the frequency of the taster variant is slightly higher in the non-African populations, the frequency of the intermediate variants is much lower, leading to an almost all-or-none PTC-tasting pattern in non-Africans. In early studies that were based on an all-or-none approach, the Africans who carry the "intermediate taster" genes would by testing have been considered tasters, hence the higher frequency of tasters in the African data mapped in the sidebar.
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 may be a useful tactic for discovering how taste sensitivity relates to other activities of the brain and mind.