The Genetics of Taste: Using PTC tasting ability to explore the connections between
genotype and phenotype
Dr. Lynn Miller, Hampshire College and Kathy McCarthy, Amherst Regional High School
Amherst, Mass
Overview: These lessons are designed to transition between understanding inheritance patterns
and the basic structure and function of DNA, to phenotypic expression of traits, accomplished
through transcription/translation of proteins; basically, the connection between genotype and
phenotype. The focus in on the connection between the TAS2R38 gene which is expressed
through the phenotypic ability to taste the bitter compound PTC (phenythiocarbamide). The
activities will explore the inheritance patterns of PTC tasting ability, the location and sequence
of the TAS2R38 gene, the processes of transcription and translation, and will culminate in a lab
that uses current genetic biotechnology to analyze each student's genotype for PTC tasting
ability. [Note: Although the final lab will initially be carried out in Hampshire College’s science
lab, the kit to run this lab is commercially available and can be done in a high school lab setting.]
This series of lessons is designed to guide students, primarily through inquiry-based lessons,
to a better, deeper understanding of the connections between the DNA molecule, genes, the
products of those genes, and the affect of the products on an individual. When students have a
good understanding of these connections, they can then begin to explore what might mediate
those connections. Extensions of these lessons could be in areas such as epigenetics,
transcriptional control, post-transcriptional control, and post-translational control of genes.
Target audience: The first run of these lessons will be used with 2 different Advanced
Placement Biology classes (juniors and seniors). After having experience with the sequence,
modifications will be made for use with introductory College Prep Biology classes.
Misconceptions:
Genes (themselves) are responsible for the manufacture of proteins
Dominant genes provide "better" traits
Mutations only produce diseases
"Genes" are dominant or recessive (genes - alleles are often interchanged)
Learning Standards (Massachusetts Frameworks):
Biology
3.2 (Describe the basic process of DNA replication and how it relates to the transmission and
conservation of the genetic code). Explain the basic processes of transcription and translation,
and how they result in the expression of genes. Distinguish among the end products of
replication, transcription, and translation.
3.3 Explain how mutations in the DNA sequence of a gene may or may not result in phenotypic
change in an organism.
1
Prior knowledge necessary for this sequence of lessons:
a) Inheritance patterns - both Mendelian and non-Mendelian
b) DNA structure and function
c) RNA structures and functions
Major Science Concepts Addressed:
What is gene expression?
DNA and RNA structures and functions
Processes and products of transcription and translation
Mutations vs. polymorphisms
Phenotypes and genotypes
Gene expression
Learning Objectives:
Students will be able to
⋅ distinguish between DNA and RNA
⋅ explain the processes of transcription and translation
⋅ describe the product of transcription and translation
⋅ explain how protein production can be stopped, changed, or modified in different
steps of transcription and translation
⋅ identify the difference between a mutation and a polymorphism in a gene
⋅ interpret electrophoresis gels
⋅ run a PCR-electrophoresis lab
⋅ interpret and apply data to predict phenotype from genotype
Duration: Approximately 5 regular class periods (55 minutes each) and 2 extended lab periods
(90 minutes each)
Materials:
1. Variety of packaged foods that include sour, sweet, bitter, and salty foods
1. Student directions and worksheet for Taste Sensation Lab (Appendix Item A)
2. Predicting PTC inheritance worksheet
3. Transcription-translation activity directions
4. Manipulatives for transcription-translation activity
5. Copies of "Convicted by Juries, Exonerated by Science: Case Studies in the Use of
DNA Evidence to Establish Innocence After Trial", opening comment by Attorney
General Janet Reno (Appendix Item B)
6. Understanding Gel Electrophoresis Lab (Appendix Item C)
2
7. Equipment and materials for Gel Electrophoresis Lab
8. Student information sheet for computer resources (Appendix Item D)
9. Using Single-Nucleotide Polymorphism to Predict Bitter Tasting Ability Student
Protocol
10. Lab write-up instructions
Description:
Day 1: Do you have good taste?
[Engage / Explore]
1. Teacher will have a variety of packaged foods on display for students. Students are asked to
describe the different types of taste associated with each food. (Note: packages will NOT be
opened or tasted, they are for discussion purposes only)
2. Inquiry Questions:
1. What different things can you taste?
2. Does everyone taste things to the same degree? Why or why not?
Teacher should guide students to make some connections between taste bud receptors and
genetic instructions that produce the taste buds and taste sensations.
3. Students will complete the Taste Sensation Lab and the analysis questions.
4. Teacher reviews some students answers to analysis questions and presents the following focus
questions: Does your genotype affect your ability to taste a bitter substance? Could you predict
your phenotype based on your genotype?
5. The teacher will provide brief explanation of the inheritance patterns of tasting PTC; students
will complete the Predicting PTC inheritance worksheet.
Day 2: What gives you your sense of taste?
[Explain]
1. Inquiry Questions: (to be discussed separately)
1. What gives you your sense of taste?
2. How do you get from genotype to phenotype?
2. Question #1. This is a good question to talk about as a group, as the teacher will probably
have to guide students toward the ultimate creator of taste – the genes.
3. Question #2: The teacher might precede discussion of this question by having a
representation of the cell (e.g. on board, overhead, ppt) to which students can refer. Teacher,
again, may have to ask guided questions to focus students on important parts of the cell.
3
3. Teacher presents a lesson on transcription/translation. [Magnetic white board manipulatives
from Science Kit/Boreal Labs – Protein synthesis chalkboard model - are a great (optional)
resource].
4. (Optional – could be homework) Students will complete on-line transcription-translation
http://learn.genetics.utah.edu/content/begin/dna/transcribe/
activity :
5. Students will complete a hands-on transcription-translation activity from
http://serendip.brynmawr.edu/sci_edu/waldron/#trans Scroll down to “From Gene to Protein
activity. Great handouts and instructions.
Day 3 (extended period): How can you determine a person's genotype?
[Explain]
1. Inquiry Questions:
1. Why would you want to know a person’s DNA or genotype?
2. Do we have ways of determining a person’s DNA or genotype? What are they?
2. Discuss inquiry question 1. Encourage discussion about current TV medical dramas or cases
in the news.
3. Hand out copies of article "Convicted by Juries, Exonerated by Science…" in Appendix B for
for students to read. Ask students how DNA is analyzed in order to convict or exonerate
suspects in a crime?
4. Teacher gives a brief explanation of how small pieces of DNA from several sources
are run through gel electrophoresis, then the pattern of DNA fragments are compared for
matches.
3. Students are given the "Student Guide" accompanying the "Understanding Gel
Electrophoresis Lab" {Appendix C}, and asked to review the objectives and background.
4. Teacher gives directions for lab - students complete lab and answer analysis questions.
Day 4: Practicing PCR and Gel Electrophoresis through virtual labs {access to computers
Necessary} {Websites in Appendix D}
1. Give students the student information sheet for computer resources
2. Students will run through the virtual Electrophoresis lab as a review from the previous day.
3. Students will then run through the virtual PCR lab.
4. Students should read through the background information on PTC testing.
4
For homework, students should be given a copy of the PTC tasting lab protocol to read and be
prepared for the next day. {Appendix E}
Important note: If you have access to a PCR thermal cycler and gel-electrophoresis equipment,
this lab can be done entirely in a high school classroom
Day 5: Beginning to make connections
Explain/Extend
1. Part 1 of the “Using a single-nucleotide polymorphism to predict PTC tasting ability” will be
conducted in the high school science lab.
2. Students will take samples of their DNA from cheek cells, step I of the lab protocol.
3. Students will amplify their DNA using a PCR thermal cycler. This step will begin during
class; the machine will finish running through the cycles and the DNA will be kept on ice until
beginning the next step.
4. The remaining parts of the lab will be conducted at the Biology lab at Hampshire College.
Day 6: Digesting the DNA
1. Students will take their products from the previous lab and carry out a digest of the DNA
with HaeIII restriction enzymes.
2. While students are waiting for this step to finish, they will use the college computers to
complete a sequencing activity. This activity will help identify the base sequence of the
TAS2R38 gene and compare the human gene with the same gene in other organisms.
Gel Electrophoresis
1. Students will use their restriction digest products and run a Gel electrophoresis
2. The resulting gel will be photographed, and students will make comparisons between
different DNA samples, identifying the types of alleles making up their genotype for tasting
PTC.
3. Based on their identified genotypes, students will then predict if they are a “taster” or a “nontaster”. Students will be given plain paper and PTC coated paper to test their phenotype.
4. Class will discuss the results and draw conclusions based on looking at genotype and
phenotype. Discussion should include other factors that may affect phenotype.
5
Evaluation/Extension
Students will write a research paper that extends their experience with bitter taste:
Which gene(s) influence one other type of taste receptor– sweet, salt, sour, or umami?
Students will report current information about the identity of the gene(s) that affect(s)
these taste receptors and some of the different phenotypes associated with gene(s).
The paper will present a claim about one of the other taste receptors, evidence to support
that claim, and the reasons that the evidence, in fact, support the claim.
{Grading rubric – Appendix F}
6
APPENDIX
A: Investigating the Sense of Taste
Materials:
Each group of two students will need
⋅ 4 plastic medicine cups for solutions
⋅ 8 cotton swabs
⋅ 2 plastic cups for water (or a paper plate with celery pieces)
⋅ Paper towels
⋅ 5 ml each of sweet solution, bitter solution, salty solution, and sour solution (see
preparation section for solutions)
⋅ Latex-free gloves
Procedure:
1. One member of the team acts as the experimenter and the other as the subject.
2. Number each of the solution cups 1 – 4, and put 5 ml of each numbered solution in
the appropriate cup.
3. The experimenter will put on a pair of gloves and dip the cotton swab into the first
solution and press out the excess against the medicine cup or paper towel.
4. The subject will stick out his/her tongue and allow the experimenter to place the swab
against the 5 areas of the tongue (see attached diagram).
5. The subject will record if a taste is noticed and what type of taste it is in the data
table.
6. After the first solution is tested, the subject either rinses out his/her mouth or chews a
piece of celery to clean the palate.
7. Discard the swab that was used.
8. Repeat steps 2 through 6 for the other three solutions, one solution at a time.
9. The experimenter and the subject change places and repeat steps 2 – 9.
Observations:
Solution Number
1
2
3
4
Taste? (yes or no)
Type of taste noted
7
Analysis questions:
1. Did your tongue respond to the 4 taste sensations in more than one area? Explain
2. Were your lab partner’s results the same as yours? Explain
3. Where do you think there may be experimental errors in this lab?
4. How do you think this lab could be improved?
5. What other types of taste sensations do you think there might be? How would you test
them?
**Preparation of Solutions:
⋅
⋅
⋅
⋅
Sweet: 2 tsp. sugar + 250 ml water
Sour: 30 ml vinegar + 30 ml water
Salty: 2 tsp. salt + 250 ml water
Bitter: 2 aspirin* + 250 ml water
* you may want to substitute a different bitter solution
[NOTE: You should check to see if a parental permission slip is necessary for students to taste
materials in the lab. IF you use aspirin, you definitely need to have a parental permission slip.
You must be aware of student allergies in your classroom]
B: Electronic source for article: “Convicted by Juries, Exonerated by Science: Case
Studies in the Use of DNA Evidence
http://www.ncjrs.gov/txtfiles/dnaevid.txt
(NOTE: This is a 124 page article – I used the first
2 pages – the introduction by Janet Reno)
C: Understanding Agarose Gel Electrophoresis Lab: Prepackaged kit from Neoscience
http://www.neoscience.com : Item #20-3303
8
D: Student Computer Information Services
Virtual Gel Electrophoresis:
http://learn.genetics.utah.edu/units/biotech/gel/
Virtual PCR Lab:
http://learn.genetics.utah.edu/units/biotech/pcr
PTC tasting background information:
http://learn.genetics.utah.edu/units/basics/ptc
NPR story on the background of different tastes: “Sweet, Sour, Salty, Bitter, and….Umami
http://www.npr.org/templates/story/story.php?storyid=15819485
Article about geographical patterns associated with taste: “The geography of taste”
http://www.tastescience.com/abouttaste4.html
E. PTC tasting lab protocol
A kit with supplies and a wonderful introduction and directions is available from Carolina
Biological Supply { Using a Single Nucleotide Polymorphism (SNP) to Predict
Bitter Tasting Ability} This lab was developed through the Dolan DNA Learning Center at
Cold Spring Harbor Laboratory.
The following is a modification of that lab for our particular situation and equipment:
PTC Tasting Ability Student Protocol 4/5/09 MRN (modified protocol from Carolina Kit)
Isolating Your DNA by Saline Mouthwash
Each student will have a station with a 15mL centrifuge tube containing 0.9% Saline solution
(NaCl);
a permanent marker, paper cup; micropipets and tips, 1.5mL microcentrifuge (µfuge) tubes and a
µfube tube with 100µL of Chelex.
1) Use the permanent marker to label two 1.5mL µfuge tubes with your name or assigned
number;
2) Pour the saline solution into you mouth and swish vigorously for 30 seconds; you may scrape
your cheeks with your teeth to get more cheek cells;
3) Expel saline into paper cup;
4) Swirl the cup gently; using a 1000µL pipetter and tip transfer 1000µL of your wash into your
labeled
µfuge tube;
9
5) Place your µfuge tube in the µcentrifuge; tubes must be balanced; spin full speed 90 seconds;
6) Pour off the supernatant fluid from your tube into the paper cup; be careful not to disturb the
cell
pellet at the bottom of the tube; [There will be some left over fluid in your tube.]
7) Set a µpipet to 30µL; resuspend the cells by pipetting gently in and out;
8) Add your resuspended pellet from tube into a µfuge tube containging 100µl of Chelex
suspension; take to boiling water bath;
9) Boil in boiling water bath for 10min;
10) After boiling, shake tube vigorously for 5 seconds;
11) Spin for 90 sec in mini µfuge;
12) Pipet 30µL of the clear supernatant fluid into a labeled 1.5mL tube- this is your DNA sample
to set
up PCR reactions- save in ice or @ -20C .
II. Amplify DNA by PCR
Obtain a PCR tube containing a Ready-To-Go PCR Bead. Label with assigned number.
1) Use a fresh tip to add 22.5 µL of PTC primer/loading dye mix to the tube. Allow bead to
dissolve for 1 to 2 minutes.
2)With a fresh tip add 2.5 µL of your cheek cell DNA directly into the primer/loading dye
mix.
Make sure no cheek cell DNA remains on the tip after transfer.
3) Store sample on ice until class is ready to begin thermal cycling.
4) Place PCR tube in programmed thermal cycler .
Method: taste-64c
32 cycles
Denaturing step: 94C 30 seconds
Annealing step: 64C 45 seconds
Extending step: 72C 45 seconds
Final hold
4C forever
5)After cycling, store tube on ice until class is ready to continue.
*After loading the cycler, we will discuss the rest of the experiment and practice loading,
running and analyzing gels with a separate kit. The remaining steps will be preformed after
students leave, and results will be emailed at the end of the day.
III. Digest PCR products with HaeIII (This will be done after class leaves)
1. Label a 1.5mL tube with assigned number and with a “U” (undigested)
2. Use a fresh tip to transfer 10µL of PCR product into tube “U”. Store on ice until next
step.
3. With a fresh tip add 1µL of restriction enzyme directly into the PCR product remaining in
tube. Label this tube “D” (digested).
4. Mix and pool reagents by pulsing in microcentrifuge.
5. Place PCR tube in thermal cycler programmed for 37C for 60 minutes.
10
6. Store on ice or in freezer until next step.
IV. Analyze PCR products by Gel Electrophoresis
The agar used in this part of the experiment contains Ethidium Bromide. Use gloves and
reasonable care when handling.
1) Obtain small gel form with an 8 well comb.
2) Level the device and pour 40mL 2% agar to a height about 1/3 up the comb.
3) Allow gel to harden for at least 20 minutes.
4) Remove the comb carefully and place the gel into the electrophoresis chamber.
5) Add 1xTBE to both sides of the chamber and stop filling after the gel surface is covered and
wells are submerged.
6) Use a µpipet with a fresh tip to load 20µL of pBR322/BstNI size markers into the far left
lane of the gel.
7) Using a fresh tip for each, add 10µL of the undigested (U) sample into one well and 16µL
of the digested (D) sample into another.
1
Size
marker
2
3
4
5
6
7
8
Student1 Student1 Student2 Student2 Student3 Student3 Empty/
U
D
U
D
U
D
control
8) Run the gel at 160 V for approximately 30 minutes.
9) View the gel in the Gel Doc apparatus and photograph it.
* Loaded samples may be of smaller volumes, to accommodate running a second type of gel in
addition to the typical agar one.
11
Appendix F:
Project Report Rubric
basis of taste
Component
CLAIM
A conclusion that
answers the
original question
EVIDENCE
Scientific data
that supports the
claim. The data
needs to be
appropriate and
sufficient to
support the claim.
REASONING
A justification
that links the
claim and
evidence. It
explains why the
data can be used
as evidence.
(Extra credit)
REBUTTAL
Recognizes and
describes
alternative
explanations,
provides counter
evidence, and
reasoning for why
alternative
explanation is not
appropriate
2
Makes an accurate
and complete claim
about the
relationship
between gene and
type of taste
Provides
appropriate and
sufficient
evidence to
support the claim.
Evidence is from
reliable source(s).
Provides reasoning
that links evidence
to claim. Includes
appropriate and
sufficient
scientific
principles.
Recognizes
alternative
explanations and
provides
appropriate and
sufficient counter
evidence and
reasoning.
LEVEL
1
Makes an accurate
but incomplete
claim
Provides
appropriate, but
insufficient
evidence to
support claim or
has sufficient
evidence, but is
unreliable.
Provides reasoning
that links claim
and evidence, but
repeats some
evidence and/or
includes
insufficient
evidence.
Recognizes
alternative
explanations and
provides
appropriate, but
insufficient
counter evidence
and/or reasoning.
Genetic
0
Does not make a
claim, or makes an
inaccurate claim
Does not provide
evidence, or only
provides
inappropriate or
unreliable evidence.
Does not provide
reasoning or only
provides reasoning
that does not link
evidence to claim.
Recognizes
alternative
explanations, but
does not provide
evidence or
reasoning in
rebuttal.
12
13
Kathy McCarthy and Lynn Miller
Reflection and follow up to project
Before getting into the details of our project, I would like to say that this collaboration
worked extremely well and the results were exciting and encouraging. I felt we were both
prepared, organized, and had a clear idea of what we wanted to accomplish. The focus, and most
important part of this project, was the students. To that end, the high school students had a
unique lab experience, an opportunity to work with college students, and chance to apply
classroom knowledge to real-world problems.
The most difficult piece of the puzzle was getting our schedules to mesh. Hampshire
College runs on a semester system, while Amherst Regional High School is on a trimester
system. That means the AP Biology course, for which the project was originally designed, meets
for only 2 trimesters, or until early March. The genetics unit happened to fall during Hampshire
college’s holiday break. By the time students were back and we could start scheduling, the AP
class was nearing the end and hard pressed to find any extra time to dedicate to this lab.
We were very persistent, however, and were finally able to find a compatible date in late April.
Our second obstacle was how to choose students to participate. The AP class was over,
and I was teaching 3 College Prep level (our lowest level) Biology classes. There were also
concerns about the number of students Hampshire College could accommodate at one time. I
decided to choose a few of my top students in each of my classes, plus a couple students from
another AP Biology class to participate. This turned out to be one of the best parts of the project,
as normally overlooked students were recognized for their effort, and we could see how the lab
could be adapted to other academic levels. Several of these students were not necessarily
planning on a future in a science-related field, but they were able to see another side of science.
At the very least, they will be better-informed adults, and perhaps it may have ignited a spark of
interest in genetics.
Our original plan was to complete part of the lab at the high school and part at Hampshire
College. Due to lack of time and equipment, we adapted the lab for using only the Hampshire
College facilities. Some of the time consuming parts of the lab, such as the digests, were done
by Hampshire College student lab assistants after the high school students left, and results were
e-mailed. This worked fine, as students were able to practice techniques, set the steps, and get
their results. We used their results in discussions about genotype and phenotype.
We are planning to continue this collaboration into the future. Certainly there are details
to work out, especially in terms of timing, but it was a great success. The most important part of
the collaboration was the excitement and learning opportunities for both the high school and
college students.
PTC experiment performed 4/17/09
ARHS student
1 Surya Murty
2 Laura Woodbridge
3 Amanda Glaszcz
4 Olivia Holcomb
6 Otis Rowell
7 Ezra Ward
8 Emma Ayers
11 Tristan Kovacs
13 Laura Bustamante
14 Spencer Kaye
Taste-Test
No
Yes
No
Yes
Yes
Yes
Yes
No
Yes
Yes
Genotype (see gels)
Non-taster
taster
Non-taster
Homozygous taster
taster
taster
SORRY pcr did not work
taster
taster
taster
Many of the bands are very faint, and the DNA samples are at different concentrations, the photographs
of the gels are the best for all. Most of the genotyping that seems unclear was actually visible when the
contrast etc… was manipulated.
Crime simulation (gel run at 150V for 60 minutes)
The evidence “collected” was digested with several standard enzymes, these enzymes produce a DNA
finger print unique enough to connect or eliminate a suspect.
Two suspects – X and –Y had DNA collected and digested with the same enzymes. The digested
products were run on a gel by Kathy MacCarthy’s students. Loaded as follows:
1
2
3
4
5
6
7
8
X-1
X-2
Y-1
Y-2
E-1
E-2
Size
Empty
marker
Based on the gel:
It appears that Suspect X can be eliminated. Suspect
Y may be guilty.
PTC–PCR/ Digest results
Two gels loaded as follows:
1
D-1
2
U-1
3
D-2
4
U-2
5
D-3
6
U-3
7
D-4
8
U-4
9
D-6
10
U-6
11
D-11
12
U-11
D-Means digested with HaeIII restriction enzyme
U-Means undigested portion of DNA to use a control
1
D-7
2
U-7
3
D-8
4
U-8
5
D-13
6
U-13
7
D-14
8
U-14