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PTC--To Taste or Not to Taste
This teacher guide is provided to give sample answers to questions. Most of the questions
are open-ended, so students may have correct answers that aren't included in this guide.
Finally, although the experiment is set up to yield one correct answer, there are variations in
data between students. As long as students examine their data carefully and can justify their
answers based on their data, that's science! Data are always right and there isn't necessarily
a 'right answer'.
This electrophoresis activity would be best conducted after the PTC tasting activity. It does
not need to be conducted immediately following the PTC tasting trait activity.
In 1931, a chemist named Arthur Fox sat down and started to measure some chemicals.
Pouring hastily, some of the powdery chemical ballooned into the surrounding air. Fox’s colleagues,
all standing a few feet away from him, complained of the bitter taste in the air due to the chemical.
Fox was perplexed- he tasted nothing.
Since that day in 1931, phenylthiocarbamide- or PTC, has been used to show genetic
variation in tasting abilities. When people taste PTC, which is a non-toxic chemical, some people
taste a strong bitterness, others taste a slightly bitter taste, and others taste nothing at all. Why do
you think Fox couldn’t taste the chemical that others could? Using genetics, we can try to understand
why some people can taste this chemical and others can’t.
Why do you think some students can taste the PTC and others can’t? Are you a taster or not? This
question allows the students to start to think about what may cause traits to differ, and how
their own PTC abilities may fit into that scheme.
Jillian is a student at Cactus High School in Peoria. Her middle school class learned about PTC
tasting when her class learned about traits. As it turned out, she was not a taster. In high school,
Jillian decided to get some PTC paper and have her family do the taste test, and draw a family tree
based on the tasting data. Surprisingly, everyone in her family is a taster, her mother, her father,
both her brothers, even her grandparents and aunts and uncles. Jillian was quite perplexed. Is it
possible that Jillian cannot taste PTC when everyone else in her family can taste? Based on
what you know about genetics come up with a hypothesis and how you could test this
hypothesis.
Jillian of course could be the only person in her family to have both of the recessive genes
and therefore not a taster. If the students do not come up with that as a hypothesis, guide
them to look at the genetics of the PTC gene for Jillian and her family. One hypothesis that
students may come up with is that Jillian is not really related, using electrophoresis and
looking at the gene will allow you to confirm parentage as well as look at the genetics of PTC.
Have the students predict what they would get based on their hypothesis. If the students
come up with the idea of Jillian being recessive have them draw punnett squares.
Alternatively you can have them draw the punnett square at the end of the lab.
Some questions to get you thinking about your lab:
What is DNA and what does it do?
Deoxyribonucleic acid makes you who you are, codes for proteins, etc.
What are chromosomes and how many copies do you have in each of your cells? Where did they come
from?
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Chromosomes are DNA packaged with proteins such that it will fit into the nucleus of the cell. You
have 2 copies of each of the 22 autosomal (non-sex determining) chromosomes and either 2 X’s, if
you are female, or an X and Y, if you are male. One entire set of 23 chromosomes (22 autosomal+1
sex chromosome) came from your mother and an entire other set from your father.
How much DNA do you share with each of your parents? How much DNA do you share with your
siblings?
Parents: 50%
Siblings: Between 0% and 100% (baring in mind that all humans are 99.9% the same on a DNA
level, these numbers are DNA that you received directly from your parents and that your siblings
received directly from your parents.)
What are some characteristics or properties of DNA?
Double helix, double stranded, negatively charged, colorless, unique sequence in each individual
The most common PTC gene mutation (resulting in the inability to taste PTC) in the US population is
due to a deletion of part of the gene, which is easily tested for and visualized by DNA electrophoresis.
Explain how this type of mutation can be easily seen with electrophoresis.
Materials:
Your DNA sample
Tris-acetate/EDTA solution (TAE)
0.8% agarose in TAE
Micropipette/tips
Electrophoresis apparatus
Procedure:
1. Get your electrophoresis apparatus. Make sure the comb is in place near the BLACK
ELECTRODE and that there are stoppers at both ends of the gel space. Why near the black
electrode? DNA is negatively charged, so to move the DNA into the gel with electricity, the
DNA needs to be loaded on the negative or black side, it will then move towards the red.
2. Pour hot agarose into the gel space until it reaches the top of the gel box. Let the agarose harden,
which should take about 10 minutes. Don’t touch/move your gel until it’s hard. Why not? If the
agarose moves while it's hardening, it will harden unevenly, making it more difficult for the
DNA to move through evenly.
3. When the agarose gel is hard, take out the stoppers and gently remove the comb. Label which samples
will be placed into which wells before you add DNA to the gel.
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4. Load your DNA SAMPLES into the wells, be sure to keep track of which samples you loaded in
which lanes.
5. Now pour TAE solution over your gel CAREFULLY so that is it completely covered plus a little more.
What do you think the TAE solution is for? TAE is like saltwater - it conducts electricity.
6. Run that gel!! Plug the electrodes into your gel box (red to red, black to black), being careful not to
bump your gel too much. Plug the power source into an outlet and set at 150 V. How can you tell
your gel is running? It bubbles at the electrodes. This is a redox reaction, forming H2 gas at the
black electrode and O2 gas at the positive electrode.
Analyzing Your DNA Data
Let gel run 10-15 minutes, then carefully remove gel on gel tray and slide onto an index card for
white background. Trace a picture of the gel and tape it in the space below.
How many bands of DNA do you see in Jillian’s sample? The bands are different sized pieces of DNA,
so how many different sized pieces of PTC DNA does Jillian have? Based on what you know about
genetics, what conclusions can be made from this data?
Jillian has one band, or one sized DNA pieces, remember that there are millions of pieces of
the same size in this band in order for us to be able to see it. We cannot make many
conclusions from this data alone, except that since we are looking at (and made copies of) an
individual gene (PTC gene), we would expect only one sized piece of DNA.
Since Jillian is a nontaster, and her mom and dad are tasters, do their DNA patterns differ, if so how?
What can you conclude from this data?
Now things get a little more interesting. Jillian’s mom and dad both have two sized pieces of
DNA, one (the smaller) that is the same as Jillian’s and one that is larger since it is closer to
the wells. It seems like they have two sized pieces for the PTC gene, and each only gave the
smaller piece of DNA to Jillian.
The most common PTC gene mutation (resulting in the inability to taste PTC) in the US population is
due to a deletion of part of the gene, which is easily tested for and visualized by DNA electrophoresis.
Explain how this type of mutation can be easily seen with electrophoresis.
We know there is a genetic difference between people who can taste and people who can’t.
The gene for tasting PTC (and other similar compounds) is a specific gene that codes for a
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taste receptor (protein). Some people have the normal genes, and some people have a
modified version of the gene that is missing some nitrogenous bases, making the DNA
segment shorter. First, we make a copy of the region with the size difference, and then we
separate out the pieces (since you have one piece from Mom and one piece from Dad you
should have two different copies, although you could have received the same sized piece
from both parents, making you homozygous) using Agarose Gel Electrophoresis. The DNA
will migrate in an electric field, because it is charged (- charge therefore it will migrate
toward the +), the agarose will offer the DNA resistance, bigger pieces have a harder time
migrating than smaller pieces, and therefore do not migrate as far into the gel.
Based on the DNA gel, can you assign which DNA band corresponds to the normal PTC gene (which
you must have to taste PTC)? Which DNA band corresponds to the mutant PTC gene?
Since they both can taste, you could conclude that Jillian’s mom and dad are heterozygous for
the PTC gene, each have a copy that functions (the larger band), and a copy that is inoperative
(the smaller). They both gave Jillian their copy of the inoperative gene and therefore she
cannot taste! Remember that most of the PTC mutations found in America are actually a
deletion in the PTC gene, some of the gene is missing, therefore the gene product, the protein,
does not do it’s job. In this case the protein is a taste receptor.
Compare Jillian’s Mom and Dad’s PTC genes to Jillian’s? Based on the data, is your hypothesis
supported or refuted as to why Jillian is not a PTC taster and Jillian’s parents are PTC tasters? Draw
a Punnett square to explain your results.
Depending on their hypothesis this answer will vary. Make sure their data is used to support
or refute their hypothesis. If their hypothesis was that Jillian could not be the child of her
parents, then their data does not support this hypothesis. In the end they should see that
Jillian is got the double recessive PTC mutation of which both of her parents are carriers.
Can you predict which PTC gene Jillian got from her mom and dad?
Yes, Jillian got the recessive, p, from her mom and from her dad.
Are the DNA patterns seen in Jillian’s brother’s samples consistent with their ability to taste PTC?
They are both tasters and have at least one PTC tasting gene (P). B1 received P from both his
mom and his dad, whereas B2 is heterozygous, so he could have gotten P from mom and p
from dad or P from dad and p from mom.
What percent chance of Jillian’s parents having another baby who cannot taste PTC? How about one
that is like B1, homozygous for the PTC taster gene? Or like B2 heterozygous?
25% chance of having another child who cannot taste (both genes recessive pp), 25%
chance of having a child who is homozygous for the taster gene PP, like B1, and 50% chance
of having a child like B2 who is heterozygous. There are two different ways of coming up
with a heterozygous child, the child could get P from mom and p from dad or P from dad and
p from mom.
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Based on what you have learned from this analysis, fill in as much of the family tree as you can. If
you do not have the genotype data, fill in what you can with the phenotype data.
Based on the electrophoresis data, you can fill in Jillian’s immediate family results. However,
all we know about the rest of the family is that they can taste, therefore they must have at
least one PTC tasting gene. We do not know if they are heterozygous (Pp) or homozygous
(PP) for the PTC gene.
As an extension, Jillian could find out whether her Aunt or Uncle’s children can taste PTC,
and depending on those results she may be able to fill out more of her family tree.