I am a huge supporter of project-based learning. I was pleased when I found an interesting opportunity published in the journal American Biology Teacher. Titled “Tasting Phenylthiocarbamide (PTC): A New Integrative Genetics Lab with an Old Flavor”, this activity-oriented learning experience uses PTC tasting to illustrate an array of genomic concepts.

PTC tasting has been used as an example of a Mendelian trait controlled by a dominant gene. This paper shows how a teacher can go beyond Mendelian genetics to:

• demonstrate pedigree analysis,
• make Hardy-Weinberg calculations,
• determine student genotypes through the use of cleaved amplified polymorphic sequence analysis, and
• encourage students to examine evidence for evolution at the molecular level.

But before implementing this lab in your classroom there are some issues to consider. Since PTC is a toxic substance, its permitted use in the classrooms of Alberta has gone through a range of regulatory acceptance. Science teacher resource books published in the 1960s gave recipes for teachers to prepare their own PTC paper (A Sourcebook for the Biological Sciences – Morholt Brandwein and Joseph). Throughout the 1970s to the mid-2000s PTC was not permitted in many Alberta schools. Although it is still not included as a part of the Alberta Program of Studies, a 2006 Safety in the Science Classroom once again spelled out that PTC paper could be used under extremely controlled circumstances. PTC is classed D1 (materials causing immediate and serious toxic effects) in WHMIS. Furthermore, it classifies PTC under class B – Chemicals, i.e., not appropriate for student use in schools except in junior and senior high school science courses under highly controlled conditions of use. To be in compliance, a teacher would need to hand the paper directly to each student, “check that students do not have bleeding gums or open wounds in the mouth”, and collect the used papers into a biohazard container for disposal.

Teach.Genetics, a Genetic Science Learning Centre based at the University of Utah, indicates that many schools have (in their opinion) needlessly banned the use of PTC paper in schools based on toxicity reports. They suggest PTC is no more dangerous than table salt. They include various calculations to justify their stance. Regardless, salt (NaCl) has a median lethal dose (LD₅₀) of 3000 mg/kg whereas PTC is 3 mg/kg. This is not a trivial difference: it is several orders of magnitude.

Here are my questions:

• Were we in Alberta previously far too cautious?
• Are we now being far too imprudent?
• Is it even wise for a teacher to suggest that students taste something described on Wikipedia as being twice as toxic as potassium cyanide? (I’m imagining the hushed tones of the breathless reporter on the evening news as I type this).

Please take the time to let me know what you think of using PTC in the classroom.
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