Materials Included In Kit

Safety Precautions

It is important to know the facts about PTC paper when making a decision about its use. PTC paper is paper soaked with phenyl thiocarbamide (PTC). The LD₅₀ (Rat) for PTC is 3 mg/kg. Such a low LD₅₀ suggests that this is a very toxic substance. The solution to make the taste test paper contains approximately 50 mg of PTC per liter of water. Through some very crude arithmetic we calculate that each strip of PTC paper would contain approximately 0.03 mg of PTC, and that a person would have to lick and ingest 5000 2" x ¼" strips of PTC paper to reach the LD₅₀ for a body weight of about 50 kilograms (110 pounds). A few sensitized individuals may have an allergic reaction to PTC. You will have to judge whether to use this taste test paper.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Dispose of test papers following Flinn Suggested Disposal Method #26a.

Teacher Tips

• Enough materials are provided in this kit for a class of 30 students. In order to have enough PTC paper to allow the testing of family members, students can cut the strips into several pieces for testing. The main part of the lab can be completed in a 50-minute class period. Collection of data for Part I is best done over a weekend. This will increase the probability that students can test grandparents, etc. Additional class periods should be allotted for pedigree analysis discussions as appropriate.
• Be very sensitive to family histories whenever doing genetic data collection. If genetic irregularities are revealed, be sure to entertain many alternative explanations for the data. Adoptions, remarriages, unknown parentage and many other explanations are possible.
• Discuss the results of PTC testing after Part I before going on to Part II. Analyze student family pedigrees and establish the genetics of PTC.
• The simple genetic patterns to look for when analyzing pedigrees for simple dominant and recessive traits are as follows:
  
  {10250_Tips_Figure_2}
  Pedigrees where all individuals have the trait or all individuals do not have the trait are not very revealing.
• Students can construct family pedigrees for thiourea and sodium benzoate, but will likely get results that are complicated compared to PTC. For an advanced group of students this might be a challenge.
• The gene that primarily affects the ability to taste PTC is called TAS2R38. It is part of a family of genes called TAS2R all of which are related to the ability to taste bitter substances. TAS2R38 is located on chromosome 7, location q35–q36. Three amino acid substitutions are the typical differences between tasters and non-tasters. Other TAS2R genes play a minor role in the ability to taste PTC. Certain TAS2R genes contribute to those individuals called “super-tasters.”
• One study found that many people who react strongly to PTC do not like to eat foods, such as broccoli or coriander, nor are they likely to smoke cigarettes or drink coffee or tea.
• The ability to taste PTC is present in about 70% of the overall human population, varying from 58% for Australians to 98% for Native American populations.
• The genetics of taste is very complex. The ability to taste PTC seems to be a straightforward dominant/recessive situation. The genetics of tasting thiourea and sodium benzoate seem much more complex. Both seem to have a genetic basis, but are not as easily analyzed. If students conclude that the ability to taste is very complex, you will have been successful. It is another clear example of genetic variation and complexity.
• PTC results are often used to illustrate population genetic principles, such as the Hardy-Weinberg Law. By combining all the data from all classes’ pedigrees, a fairly large gene pool would be sampled and gene frequencies can be calculated.

Answers to Questions

Answers will vary from classroom to classroom as the genetic distributions change with the genetics of each demographic.

Introduction

Why are some people very sensitive to the taste of spicy food while others are not? Is this taste sensitivity inherited? This activity will explore whether the ability to taste various substances is genetic.

Concepts

• Taster
• Homozygous vs. heterozygous
• Dominant vs. recessive
• Pedigree analysis

Background

The sense of taste in humans depends on clusters of sensors called taste buds. The taste buds are embedded in the epithelium of the tongue, and many are found on the raised papillae of the tongue. A tongue viewed in a mirror might actually look fuzzy because of the papillae. There are many taste buds on each papilla.

Unlike some other senses, the taste sensors are not neurons. At their bases, taste sensors form synapses with dendrites of sensory neurons. Instead of firing action potentials (electrical nerve impulses), they release neurotransmitters onto the dendrites of the sensory neurons, which in turn, fire the action potential to the central nervous system. In other words, taste sensation involves chemical interactions prior to nerve stimulation. A lot of chemistry occurs on the tongue and as a result its epithelium is shed and replaced at a rapid rate. Individual taste buds last only a few days before they are replaced.

It is generally accepted that humans can perceive five basic tastes: sweet, salty, sour, bitter and umami. Unami is sometimes called savory. Although each type of taste sensor is found at all areas of the tongue, certain areas seem to respond more for one taste sensation versus another. The tongue can be mapped by dipping toothpicks in different solutions and touching them to different regions of the surface of the tongue. See Figure l for generalized regions of taste often found on the surface of the tongue. The final ability to “taste” something is a complex interaction between olfactory sensors (ability to smell) and stimulation of the taste buds. When a substance is placed in the mouth it quickly disperses to all four taste areas and creates a nearly instant taste sensation. Where the taste sensation occurs is really not discernible and is of little consequence when we are actually tasting something. What is perhaps more interesting is our ability to taste or not taste a particular substance.

Several interesting chemicals have been discovered that have been used by scientists to explore the genetics of the sense of taste. One of these chemicals is PTC (Phenyl thiocarbamide). Some people taste this bitter-tasting chemical very easily while other people can’t taste it at all! People who can taste PTC are referred to as “tasters.” Those who cannot taste PTC are called “non-tasters.” Not only is the difference in ability to taste this substance interesting, but it has also been found to be an inherited characteristic. In Part 2 of this laboratory you can discover how the ability to taste PTC is inherited.

Materials

Safety Precautions

Handle the taste test papers as little as possible before use. Once any taste is detected remove the test paper from the mouth and discard. Do not reuse any test papers; always use fresh test paper for every individual. Wash hands before touching the test papers and handle them as little as possible. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part I. Ability to Taste PTC

In this part of the laboratory, your ability and the ability of your relatives to taste the chemical PTC (Phenyl thiocarbamide) will be tested. After PTC tasting is determined, a family pedigree will be constructed and tested. Then how the ability to taste PTC is inherited will be determined.

For all taste tests, use a very small piece of test paper (5-mm square). Place the paper on your tongue and move it around until you have a taste sensation or until you are convinced that you will not taste anything (about 3–5 seconds).

1. Place a piece of control taste test paper in your mouth. Mentally note any taste you sense from the control paper and remember it as a negative taste test for the rest of the test papers used in this laboratory.
2. Place a small piece of PTC taste test paper on your tongue. If you sense a bitter taste (it will be obvious), then you are a taster of PTC. If it tastes like the control paper, then you are a non-taster.
3. Record if you are taster or a non-taster.
4. Secure data for the entire class reflecting the total number of tasters and non-tasters. Record this data on the Genetics of Taste Worksheet.
5. Take enough PTC and control paper home to test as many of your relatives as possible. Ask each relative to taste the papers and determine which people are tasters and which are non-tasters. Record the results carefully in a separate data table.
6. Use the information on the top of the PTC Worksheet to draw a pedigree for your family showing tasters and non-tasters. Complete the PTC Worksheet.
7. Discuss the results of Part I with your class before completing Part II.

Part II. Other Tastes

In this part of the activity, comparisons can be made between the ability to taste PTC and the ability to taste other test substances.

1. Place a small piece of thiourea test paper on your tongue.
2. Are you a taster for thiourea? Were you a taster for PTC? Record data for yourself and all the individuals in your class on the Genetics of Taste Worksheet.
3. Answer Questions 1–2 on the Genetics of Taste Worksheet.
4. Now place a small piece of sodium benzoate taste test paper on your tongue. Does it taste sweet? sour? salty? bitter? or does it have no taste at all? Record data for the sodium benzoate test for yourself and all individuals in your class on the Genetics of Taste Worksheet.
5. Answer Questions 3–5 on the Genetics of Taste Worksheet.
6. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

10250_Student1.pdf