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PTC--To Taste or Not to Taste
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?
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 aunt and uncle. 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.
Some questions to get you thinking about your lab:
Why do you look the way that you do?
How much DNA do you share with each of your parents? How much DNA do you share with
your siblings?
What are some characteristics or properties of DNA?
Materials:
Your DNA sample
Tris-acetate/EDTA solution (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?
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?
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.
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?
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?
Analyzing Your DNA Data
Let gel run 10 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. Label which
sample is which.
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?
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?
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.
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?
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.
Can you predict which PTC gene Jillian got from her mom and dad?
Are the DNA patterns seen in Jillian’s brother’s samples consistent with their ability to taste
PTC?
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?
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.