Biology 11–12 · Year 12

Frequency of PTC tasting in a class and the allele frequencies it implies

Module 5: Heredity (Genetic Variation; Inheritance Patterns in a Population)

Practical, model not builtLow risk

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The idea

PTC tasting is often treated as a trait with dominant inheritance, and under that simplification the fraction of non-tasters gives the recessive allele frequency by Hardy-Weinberg; the University of Utah page cautions that tasters vary greatly in sensitivity and that the PTC gene has about 85 percent of the influence, so the figure is an estimate under a simple model.

What you need

PTC (phenylthiocarbamide) test paper strips from an educational supplier, one per participant, plus control (untreated) strips
Anonymous tally sheet; calculator or spreadsheet

How to do it

Explain that participation is voluntary and that only counts are recorded, never names.
Each participant tastes a control strip, then a PTC strip, and reports taster (bitter) or non-taster.
Tally tasters and non-tasters for the class, and pool with other classes if available.
Take the non-taster fraction as q squared (the recessive homozygote frequency), compute q, then p = 1 - q, and the expected genotype frequencies p squared, 2pq and q squared.
Compare the class proportion with the 75 percent taster figure the University of Utah page gives for people on average.
Discuss which Hardy-Weinberg assumptions a school class violates, and the simplification of treating tasting as one dominant gene when the Utah page gives the PTC gene about 85 percent of the influence, with two common alleles and at least five rare ones, and how both limit the estimate.

What you should see

On average about three-quarters of people taste PTC (the University of Utah figure), and a class's fraction varies widely around that. If 25 percent are non-tasters, q = 0.500, p = 0.500 and the carrier frequency 2pq = 0.500; if 30 percent are non-tasters, q = 0.548, p = 0.452 and 2pq = 0.495 (computed). These figures hold under the simple dominant model only, which the Utah page calls a simplification. The learner knows it worked when the control strip is tasteless to everyone and the class fraction lies within the range expected for a sample of its size.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Dominant means common; the taster allele is dominant and its frequency can be anything.
Everyone with the dominant phenotype has the same genotype; under the simple dominant model two-thirds of tasters in a 3 : 1 population are carriers.
A class of 28 gives an accurate allele frequency; sampling error at that size is large.

Safety card

Low riskLearners carry it out

Hazards

PTC is toxic in bulk; the paper carries a trace amount
Sharing strips

Controls

Use only supplier strips, one per person, never the powder; spit out and discard; wash hands
Voluntary participation and anonymous recording; anyone may decline
Taste only outside the laboratory, away from any chemicals

Note

PTC is not among the chemicals banned from NSW government schools: it does not appear in Appendix H of the Chemical Safety in Schools package (https://education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/may-2021/appendices/Appendix_H_-_Chemicals_which_are_banned_from_government_schools.docx), read on 23 September 2026. Work from the supplier's safety data sheet for the strips and the school's own risk assessment (https://www.riskassess.com.au/) before running it, and never open or handle the powder. Human genetic data are collected without identifiers.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO12-12 BIO11/12-4 BIO11/12-5 BIO11/12-6
Biology 11–12 Syllabus (2025), Year 12 focus area Heredity; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-12-01
Biology 11–12 Syllabus (2025), Year 12 focus area Biodiversity; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-12-03
Biology 11–12 Syllabus (2025), Year 12 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-12WS-04 BI-12WS-05 BI-12WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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