A TREE OF GENETIC TRAITS
Create a visual representation of the frequency of certain trait combinations within the class.
Prep Time: 30 minutes
Class Time: 30 minutes
Students mark their traits on tree leaf cut-outs (available in English and in Spanish).
Detailed instructions with discussion points and extensions guide you through the activity.
Download the PDF
Students place their leaves on a large tree whose branches each represent a different combination of traits.
The 2004 publication Investigating Safety: A Guide for High School Teachers by Texley et al. has raised an alarm in the teaching community about the usefulness and safety of PTC taste testing. This has led to PTC being banned from many schools and districts - we believe unnecessarily.
Yes, PTC is toxic. In rats, the most sensitive animals tested, the oral LD50 of PTC (the amount that killed 50% of test animals) is 3 mg/kg. However, PTC is so intensely bitter that tasters can detect it in miniscule quantities. A single test paper from Carolina Biological Supply contains just 0.007 mg of PTC. And the amount that is licked off the paper by a test subject is much less than this. In addition, there has not been a single report of toxicity arising from PTC taste testing, which has been performed on tens of millions of individuals worldwide.
To put the toxicity of PTC into perspective, we offer this quote (from Merritt et al., Am. Biol. Teacher online 70:4):
There is no question that PTC is toxic (LD50 in rat is 3mg/kg, in mouse 10mg/kg, and in rabbit 40mg/kg), but so is table salt (acute toxicity in humans at 500-1000mg/kg). The issue is how much PTC is on a taste paper. Texley et al. indicate that "a single strip contains about 0.3 mg" but the two suppliers we checked with indicate that a taste paper contains either 0.007 mg (Carolina Biological Supply Company) or 0.005 - 0.007 mg (ScienceStuff). Assuming a linear dose response curve, we calculate that the 230 mg of NaCl in a vending machine bag of potato chips is about 100 times more toxic than the 0.007 mg of PTC in a taste paper. We do not believe there is any reason for teachers to be concerned about the toxicity of PTC taste papers.
Funding provided by a Howard Hughes Medical Institute Precollege Science Education Initiative for Biomedical Research Institutions
Award (Grant 51000125)
Funding for significant revisions provided by contract U33MC00157 from the Health Resources and Services Administration, Maternal and Child Health Bureau, Genetic Services Branch and the March of Dimes.
Thank you to Dr. Robert Merritt and Dr. Steven Wooding for information about PTC safety.