PTC: GENES AND BITTER TASTE
In 1931, a chemist named Arthur Fox was pouring some powdered PTC into a bottle. When some of the powder accidentally blew into the air, a colleague standing nearby complained that the dust tasted bitter. Fox tasted nothing at all. Curious how they could be tasting the chemical differently, they tasted it again. The results were the same. Fox had his friends and family try the chemical then describe how it tasted. Some people tasted nothing. Some found it intensely bitter, and still others thought it tasted only slightly bitter.
Soon after its discovery, geneticists determined that there is an inherited component that influences how we taste PTC. Today we know that the ability to taste PTC (or not) is conveyed by a single gene that codes for a taste receptor on the tongue. The PTC gene, TAS2R38, was discovered in 2003.
There are two common forms (or alleles) of the PTC gene, and at least five rare forms. One of the common forms is a tasting allele, and the other is a non-tasting allele. Each allele codes for a bitter taste receptor protein with a slightly different shape. The shape of the receptor protein determines how strongly it can bind to PTC. Since all people have two copies of every gene, combinations of the bitter taste gene variants determine whether someone finds PTC intensely bitter, somewhat bitter, or without taste at all.
Although PTC is not found in nature, the ability to taste it correlates strongly with the ability to taste other bitter substances that do occur naturally, many of which are toxins.
Plants produce a variety of toxic compounds in order to protect themselves from being eaten. The ability to discern bitter tastes evolved as a mechanism to prevent early humans from eating poisonous plants. Humans have about 30 genes that code for bitter taste receptors. Each receptor can interact with several compounds, allowing people to taste a wide variety of bitter substances.
If the ability to taste bitter compounds conveys a selective advantage, then shouldn't non-tasters have died off long ago? Why do so many people still carry the non-tasting PTC variant? Some scientists believe that non-tasters of PTC can taste another bitter compound. This scenario would give the greatest selective advantage to heterozygotes, or people who carry one tasting allele and one non-tasting allele.
PTC sensitivity is often used as an example of a simple Mendelian trait with dominant inheritance. However, tasters vary greatly in their sensitivity to PTC. And while the PTC gene has about 85% of the total influence over whether someone is a taster or a non-taster, there are many other things that affect PTC tasting ability. Having a dry mouth may make it more difficult to taste PTC. What you ate or drank before sampling PTC paper may also affect your tasting ability. And an individual's sensitivity may change over time. Some people may find that they can taste PTC on some days, but not on others.
Studies indicate that individuals with the "strong tasters" PTC gene variant were less likely to be smokers. This may indicate that people who find PTC bitter are more likely than non-tasters to find the taste of cigarettes bitter and may be less likely to smoke.
Other studies suggest that there may be correlations between the ability to taste PTC and preferences for certain types of foods. This may be why some of us think that broccoli is just too bitter to eat.
Merritt, R., Bierwort, L., Slatko, B., Weiner, M., Weiner, E., Ingram, J. and Sciarra, K. (2008). Tasting Phenylthiocarbamide (PTC): A New Lab With an Old Flavor. Am. Biol. Teacher online 70:4.
Wooding, S. (2006). Phenylthiocarbamide: A 75-Year Adventure in Genetics and Natural Selection. Genetics 172 (4): 2015-2023.
Funding for this feature was provided by the Howard Hughes Medical Institute.
Special thanks to Dr. Stephen Wooding, Assistant Professor, University of Texas Southwestern Medical Center.