Download Teacher`s Guide: Genetics of Taste

Teacher’s Guide: Genetics of Taste
Objectives: (Alabama Course of Study Biology – Objective #7)
The student will be able to
o
define important genetic terms, including phenotype, genotype, homozygous, heterozygous,
dominant trait, recessive trait, and allele.
o
interpret inheritance patterns shown in graphs and charts.
o
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:
o
allele
o
dominant
o
recessive
o
genotype
o
phenotype
o
homozygous
o
heterozygous
o
Punnet Squares
o
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:
o
Teacher’s Guide: Genetics of Taste … background information
o
Student Handout: Genetics of Taste
o
Control taste strips*
o
PTC taste strips*
o
Thiourea taste strips*
o
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 openended question 10 points.
Accomodations:
o
Multiple Learning Styles are addressed through use of audio-visual equipment, mathematical
calculations, and kinesthetic activities.
o
Cooperative Learning will take place as students combine class data.
o
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 family pedigree. 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:
Student’s Mom:
Student’s Father:
Student’s Sister:
Student’s Sister:
Student’s Brother:
Student’s Paternal Grandmother:
Student’s Paternal Grandfather:
Student’s Paternal Uncle:
Student’s Maternal Grandmother:
Student’s Maternal Grandfather:
Student’s Maternal Aunt:
Student’s Maternal Aunt’s Husband:
Student’s Male Cousin From her Maternal Aunt:
Student’s Female Cousin From her Maternal Aunt:
Student’s Female Cousin From her Maternal Aunt:
taster for PTC
taster for PTC
taster for PTC
taster for PTC
taster for PTC
taster for PTC
taster for PTC
taster for PTC
non-taster for PTC
taster for PTC
non-taster for PTC
taster for PTC
taster for PTC
taster for PTC
taster for PTC
taster for PTC
Completed Student Pedigree
TT or Tn
nn
Tn or nn
Tn
Tn
Tn or TT
TT or Tn
Tn
nn
STUDENT 
Tn or
TT
Tn or
TT
Tn or
TT
Tn or
TT
Tn or
TT
Tn or
TT
Tn or
TT
Pedigree Analysis…
Because the ability to taste PTC is a dominant trait, everyone who
was considered a taster must have the genotype TT or Tn. The
genotype nn can only be assigned to those that are non-tasters for
this situation. Because the student’s maternal grandfather was a
non-taster with a genotype of nn, her mother, which was a taster,
must have the genotype Tn. The same pedigree analysis was used to
determine the student’s maternal aunt. Because the student’s
paternal uncle was also a non-taster, he was labeled with the
genotype nn. Therefore, the student’s paternal grandparents both
had to carry a recessive gene for this trait.
Key (Trait: Ability to Taste PTC)
= female
= male
or
= person that has the trait
*There are no half colored shaped to represent carriers,
since the ability to taste PTC is a dominant trait! Also,
an x has been placed through shaped of family members
that were unable to be tested.
T = Taster
n = non-taster