Supplies: 6 PTC paper tests, 4 volunteers
The following excerpt is from Genetic Science Learning Center, University of Utah. ((2016). PTC: Genes and Bitter Taste. Available from http://learn.genetics.utah.edu/content/inheritance/ptc/ (Links to an external site.)Links to an external site.
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.
Plants are much more likely than animals to contain toxins. Because avoiding bitter plants would severely limit their food sources, strict herbivores have fewer bitter taste genes than omnivores or carnivores. Instead, animals that graze on plants have a high tolerance to toxins. Grazers have large livers that are able to break down toxic compounds. PTC stands for phenylthiocarbamide. Also known as phenylthiourea, the chemical structure of PTC resembles toxic alkaloids found in some poisonous plants. 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. The ability to taste PTC shows a dominant pattern of inheritance. A single copy of a tasting allele (T) conveys the ability to taste PTC. Non-tasters have two copies of a non-tasting allele (t).
Bitter-tasting compounds that are present in cruciferous vegetables (such as cabbage, broccoli, brussel sprouts, turnips, and kale), coffee, tea and cigarettes have chemicals which are similar to the chemical structure of PTC. Individuals carrying one or two bitter-sensitive alleles have a stronger preference for sweet-tasting food and beverages compared with bitter foods and drinks.
- Find four friends to test in addition to yourself (no children under age 18). No safety issues have been reported with PTC test papers. In large doses the chemical is toxic but the papers contain an extremely small amount. However be sure no organism (people or pet) ingest a large quantity of the papers. No one should be tested more than once. Ensure that all your subjects agree to the test.
- Handle the taste test papers as little as possible before use. Make sure to have a drink of water before testing. A dry mouth can make it more difficult to taste PTC.
- Place one strip of PTC taste paper in the center of your tongue for several seconds. If you are a PTC “taster” you will taste it immediately. If you are not, remove the paper after a couple seconds. Discard used papers in the trash. Do not reuse any test papers; always use a fresh test paper for each individual. Wash hands after completing the test.
- Collect the following information from each person tested:
- Did it taste strongly bitter, weakly bitter, or no taste?
- Do you like broccoli, kale or Brussel sprouts?
- Do you prefer really sweet desserts, less sweet or no dessert?
- Enter your data into the google form (one form for each person you tested): https://docs.google.com/forms/d/1APiG-pTsltauHqKQbWTqyaHnI3P5ZB0gMwQYj7W64gU/viewform Links to an external site. No credit for the assignment if you don’t enter your data here.
Create a Word document with the following:
- Create a data table for the tests you conducted.
- Explain how natural selection would play a role in the frequency of tasters in the population.
- Overall, we are investigating the distribution of PTC tasting in the population. Create two hypotheses to investigate. They need to be answerable based upon the data the class is collecting. Make sure to make them specific, clear and testable.
- What are the dependent and independent variables for each hypothesis (previous question)?
- What are two confounders in this experiment?