Bitter Taste (Activity)

Genetics leaves a bad taste in my mouth… or not

Some of our personal preferences arise from the way we were brought up. Culture plays a role in our likes and dislikes. Likewise, our experiences play a role in how we respond to certain stimuli. Another major factor that plays a role into our preferences comes wired in our genome. The DNA in our cells is the instruction manual for who we are. We are programmed to seek out things of a nutritive values in order to acquire raw materials like carbohydrates, proteins and lipids. In our search for nutritive compounds we have learned to avoid things that don’t taste good. Bitter things have a tendency to be associated with toxic compounds in nature. When eating a food item for the first time, molecules hit our tongue and stimulate multiple sensations: sweet, sour, salty, savory and bitter. Attributed to these multiple taste types are a diverse family of receptors that bind to the molecules that result in our perception of these sensations. Something bitter might make us learn to avoid this food item in the future.
One type of bitter receptor senses the presence of a chemical called phenylthiocarbamide (PTC). This chemical chemically resembles toxic compounds found in plants but is non-toxic. The ability to taste PTC is comes from the gene called TAS2R38. This gene encodes a protein that on our tongues that communicates the bitterness of this chemical. There are two common alleles of this gene with at least five more uncommon variants. Within the two common forms, a single nucleotide polymorphism (SNP) is responsible for changing one amino acid in the receptor. It’s this difference of one amino acid that results in the ability of the receptor to either respond or not respond to PTC. We inherit one copy of the gene from our father and one copy from our mother. Based on how our parents gametes were formed and what alleles we received during the fertilization event determines how we respond to this chemical. Because we each have 2 copies of this gene, we can utilize simple Mendelian genetics to understand which allele is dominant or recessive.

  1. Place a piece of “Control” paper on the tongue and indicate if there is a taste
  2. Place a piece of “PTC” paper on the tongue and indicate if there is a taste and the taste severity
  3. Fill out the table for the class to identify how many non-tasters, tasters or super-tasters there are.
  4. Indicate if you believe the trait is dominant or recessive (ability to taste or not taste)
  5. Assign a descriptor allele for the dominant (a capital letter) or the recessive (a lowercase letter) and draw a Punnet square for the F2 generation of 2 Heterozygous parents.
  6. Compare the class tally of tasters and non-tasters in the class and discuss with your instructor if there is a clear dominance of this trait.

Table: PTC Tasting Tally

Questions:

  1. How do you explain the presence of those who can’t taste PTC, those who can taste it and those who really can’t stand the taste of it?
  2. This chemical is non-toxic and doesn’t exist in nature. Do you think there is a selective pressure that confers an advantage to those who do taste it?
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