Teacher’s Guide: Genetics of Taste

Objectives: (Alabama Course of Study Biology – Objective #7)

The student will be able to

  • define important genetic terms, including phenotype, genotype, homozygous, heterozygous, dominant trait, recessive trait, and allele.
  • interpret inheritance patterns shown in graphs and charts.
  • form an evidence based opinion through scientific experimentation.

Background:

Is your sense of taste a matter of nature or nurture? Students will examine the genetics of taste by testing their ability to taste PTC and Thiourea. After calculating the frequency of traits, students can complete an extension activity by creating a family pedigree and attempt to classify genotypes of family members. Students and teachers should both be familiar with the following terms before beginning this activity:

  • allele
  • dominant
  • recessive
  • genotype
  • phenotype
  • homozygous
  • heterozygous
  • Punnet Squares
  • pedigrees

This is the second activity from the lesson Pass It Down. Therefore, if students have not completed the first activity, Frequency of traits, students will not be as familiar with genetics terms or procedural calculations. Therefore, review the lesson plan for Frequency of Traits as needed and plan accordingly.

Materials:

  • Teacher’s Guide: Genetics of Taste … background information
  • Student Handout: Genetics of Taste
  • Control taste strips*
  • PTC taste strips*
  • Thiourea taste strips*
  • calculators

*Taste strips can be purchased from biology supply companies, such as Flinn or Carolina. These test strips are prepared with chemicals, such as phenyl thiocarbamide, which can be toxic. However, it is estimated that a student would have to consume 500 taste strips to reach a lethal dose.Teachers should always use discretion when choosing to use chemicals in class and should always follow safety rules that accompany all materials purchased from biological supply companies.

Methods:

Prior to Beginning the Activity:

Purchase test strips from a biological supply company at least one week before completing this activity. Read all attached safety precautions.

The Day of the Activity:

Anticipatory Set(Use the Pass It Down PowerPoint Presentation included!)

Is your sense of taste a matter of nature or nurture? Conduct a quick discussion style debate that forces students to question why certain foods taste good to some people and bad to other people. Students may be tempted to say that it depends on what you are used to eating and mention that foods taste better with age as you get older because your sense of taste may not be as sharp. However, try to lead them to question whether taste can also be genetically determined.

Guided and Independent Practice

  1. Make sure students have thrown away any chewing gum or cough drops that could alter their sense of taste before beginning this activity.
  2. Model appropriate safety and disposal procedures while handling taste strips. Pass a student handout and control taste strip to each student, instructing them to wait until directed to taste.
  3. Explain to students the value of using a control paper… Even though the control paper does not contain any chemicals, it will still have somewhat of a taste to each student, and they do not need to confuse the taste of the paper with the taste of a chemical later on during the investigation.
  4. Guide students to taste, but not consume, the control taste strip. Whatever is “tasted” should be considered neutral.
  5. Pass a PTC taste strip to each student, instructing them to wait until directed to taste.
  6. Explain that the ability to taste PTC is genetically determined and is a dominant trait. Therefore, some students will taste PTC, and some will not. (You should probably apologize to the “tasters” in advance, since it is not a sweet taste!)
  7. Guide students to taste, but not consume, the PTC taste strip. If they can taste the PTC, they should consider themselves a “taster.”
  8. Follow the same procedure, apology and all, for steps 5-7 using the Thiourea taste strips.
  9. Guide students in completing their calculations by reviewing procedures used in the previous activity, Frequency of Facial Traits.
  10. Dispose of all taste strips.

Conclusion:

Check student calculations to make sure the frequency between two opposing traits equal 100%. For example, if 70% of the class can taste PTC, then 30% should be considered a non-taster for PTC. Instruct students to provide evidence based explanation to back up their opinion to the open ended question at the end of their student handout.

Assessment: (50 points)

Count each cell in their data table 5 points, for a total of 40 points. Then, count the response to the open-ended question 10 points.

Accomodations:

  • Multiple Learning Styles are addressed through use of audio-visual equipment, mathematical calculations, and kinesthetic activities.
  • Cooperative Learning will take place as students combine class data.
  • Instruction is modeled for all students.

Extensions:

Students can create a family pedigree about the ability to taste PTC. However, many students in class would be unable to complete a personal familypedigree. Some students may be adopted and would have no biological link to other family members. Some students may have step parents and step siblings, which can often complicate completing a pedigree. Some students may not have a family large enough to complete a pedigree with conclusive information. BE SENSITIVE TO THESE ISSUES IN CLASS, and proceed with caution in how this extension activity is handled. I have found it best to let one student volunteer to test their family members for the ability to taste PTC (assuming they are genetically related to all members tested and have a fairly large family – the more members the better!) and allow the entire class to use that data to create a family pedigree. Send home a “control envelope” and an “experimental envelope” with the appropriate amount of taste strips in each. An example of results that could be obtained and a completed pedigree is included below. Remember, the ability to taste PTC (or Thiourea) is a dominant trait!

Example Data

This is actual data from a student volunteer used in one of my classes. However, for matter of privacy, names have been removed and only relationships have been recorded. Keep in mind that you do not need a family this size to complete this extension activity!

Student:taster for PTC

Student’s Mom:taster for PTC

Student’s Father:taster for PTC

Student’s Sister:taster for PTC

Student’s Sister:taster for PTC

Student’s Brother:taster for PTC

Student’s Paternal Grandmother:taster for PTC

Student’s Paternal Grandfather:taster for PTC

Student’s Paternal Uncle:non-taster for PTC

Student’s Maternal Grandmother:taster for PTC

Student’s Maternal Grandfather:non-taster for PTC

Student’s Maternal Aunt:taster for PTC

Student’s Maternal Aunt’s Husband:taster for PTC

Student’s Male Cousin From her Maternal Aunt:taster for PTC

Student’s Female Cousin From her Maternal Aunt:taster for PTC

Student’s Female Cousin From her Maternal Aunt:taster for PTC

Completed Student Pedigree