Introduction To Heredity

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. 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).

Activity Detail What You'll Need WaterSaltShampoo or dish soap (clear shampoo with EDTA, such as Suave Daily Clarifying, works best)Cold 70% to 95% isopropyl alcohol (place in freezer overnight)Ziploc-type bagStrawberries or other foods like grapes, kiwi, tomatoes, bananas or lettuceTest tubeCoffee filterPlastic cupPipette What to Do Create the DNA extraction buffer by mixing 450 milliliters of water with two teaspoons of salt. Gently add 50 milliliters of shampoo (this recipe is enough for 50 groups of students). Place a strawberry in a Ziploc bag and gently mash for two minutes, being careful not to pop the bag. Assemble the DNA filter by draping a coffee filter over a plastic cup. Pour a small amount (three to four milliliters) of the filtered strawberry solution into a test tube. What's Happening The shampoo helps dissolve the fatty cell membrane, while the salt makes the DNA molecules stick together and breaks up the proteins that bind to the DNA.