Experiment A: PTC Genetics


As stated above, we will be performing two different experiments over the next three weeks. The first experiment focuses on the molecular genetics of a taste receptor found on the tongues of vertebrates. Many animals have specific taste receptors that help determine if a substance is bitter. This is often useful, as bitter substances are generally correlated with some level of toxicity. For this lab, we will be working with a synthetic, non-toxic chemical called phenylthiocarbamide (PTC) to mimic naturally occurring bitter compounds.


Humans can be classified along a spectrum of tasting ability for PTC. Some people detect a strong bitterness to the substance, whereas others taste nothing. The receptor protein is encoded by an autosomal gene called TAS2R38, which contains two common alleles and several uncommon variants. Differences in the two common alleles are due to a single nucleotide polymorphism (SNP) that changes one of the amino acids. The tasting allele, T, is thought as dominant to the non -tasting allele, t. Homozygous dominant (TT) individuals generally describe the taste as very bitter, heterozygotes (Tt) may detect mild bitterness, and homozygous recessive individuals (tt) will not detect any bitterness. Although many researchers treat the genetics of PTC tasting as a simple Mendelian trait (one gene, two alleles, complete dominance), the actual genetics is probably more complex involving many genes and also including an environmental component. Thus, the ability to taste PTC is likely a polygenic trait. Even at the single gene level (TAS2R38), there is no consensus as to whether the trait exhibits complete versus incomplete dominance.


Since having the tasting allele, T, would seem like a benefit due to an organism’s ability to detect potentially toxic compounds (and avoid or adapt to them), why has natural selection not removed the non-tasting allele, t, from populations? One reason might be balancing selection due to overdominance (heterozygote advantage). Balancing selection tends to maintain multiple alleles in a population. In the case of PTC, there may be some selective advantage of possessing the non-tasting allele in the heterozygote form (similar to the case of sickle-cell anemia). Clearly, more research needs to be conducted to determine the potential benefit of the non-tasting allele.


You will use PCR to amplify a 221 bp fragment of the PTC gene to determine the genotypes corresponding with the different phenotypes (tasting ability). Before beginning the molecular component of this lab we will quantify the phenotypes of the class in the table below. Each person should taste the control paper first to make sure no bitterness is detected.


Your PTC phenotype:______________


    Class Data
Phenotype Number   Frequency
Strong PTC Taster      
Weak PTC Taster      
Total     1.0


Based on the results in the table above, does it appear that PTC tasting ability follows simple Mendelian inheritance patterns? Explain your reasoning.


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