Title/Type /

Mutations: The effect on phenotype

Summary /

Investigate the differences that occur between DNA sequence that codes for the PTC taste receptor gene and relate the information to differences in phenotype - in this case, the ability to taste PTC.

LS (9-11) 1.2: 2b & 2c / Background Information
For many years, scientists have studied how and why the ability to taste certain compounds differs from person to person. In the 1930’s scientists discovered that some people can taste a bitter compound known as PTC (phenylthiocarbamide), while others cannot. In order to taste something, a receptor on the tongue bindsi a specific chemical in food. When the chemical binds to the receptor, a signal is sent to the brain where it is interpreted as a particular taste—sour, bitter, sweet,
salty or umami.
Recently, the gene that codes for the receptor that binds PTC was identified and its DNA sequence determined. Researchers have identified three nucleotide positions that vary within the human population. Sequence differences at these three contribute to the ability or inability to taste PTC.
Materials / - computer
- projector
- worksheets and/or workstations
- PTC and control taste strips
Engage / Provide students with control taste strips. Ask students to place this first strip of paper in their mouths. Ask -
-What do you taste?
Ask students to place the second strip of paper (containing PTC) in their mouths. Ask –
-What do you taste?
Explore / Are we all able to taste a bitter compound?
How many can taste PTC?
How many cannot taste PTC?
What percentage of people in the class are tasters?
How is it possible that we taste things differently?
Engage / Ask the class: Please examine the DNA sequences from two different people: “NT” and “TR.” The sequences you are examining code for a taste receptor that allows people to taster a bitter compound – called phenylthiocarbamide – or PTC. Are the sequences identical? If not, how many differences can you identify? At what nucleotide positions are these differences located? / Worksheet with projector, or computer workstations if available.
Predict / How might the differences in DNA sequence affect the taste receptor protein?
Explore / Students will transcribe and translate DNA sequences, one from TR; the other from NT to determine if the resulting amino acids differ. / Students use worksheets for transcription activity.


/ How many differences are found in the amino acid sequences of “TR” and “NT”?
How could these differences affect the function of the protein?
How can we test if TR and NT differ in their ability to taste PTC?
Why would it be beneficial to have the ability to taste PTC?
Could it be harmful to not be able to taste PTC? Why?
Could it be beneficial to not taste PTC?
TR = taster; NT = non-taster

Please note: Several of these activities can be strung together, even within a class period; so they can be short, or not.