In 1930, Arthur Fox was synthesizing a chemical called phenylthiocarbamide (PTC). Some of it was released into the air. Fox’s colleague complained that it had a very bitter taste, whereas Fox didn’t taste a thing.
After researching it was found that the ability to taste PTC is controlled by our genes.
The specific gene has two alleles: the dominant allele (T), having the ability to taste PTC, and the recessive, non-taster allele (t).
The PTC taste receptor gene, TAS2R38, was identified in 2003.
The purpose of the lab is to test whether or not our genotype indicates our phenotype of whether we can taste PTC or not.
First, to figure out your genotype, swab the inside of the cheek to get a cheek cell sample. Then, isolate just the DNA from those cells and amplify that DNA using PCR. Using a primer to target specifically the gene identified for PTC tasting, TAS2R38, make millions of copies to study. Next, use the restriction enzyme Haelll, which acts as molecular scissors, to cut the DNA sequences at GGCC. Lastly, perform gel electrophoresis on the DNA samples to create separate nucleotide strands to determine your genotype.
The last step of the lab is to test your true phenotype of whether or not you can taste PTC. You will get a control piece of paper first and then the PTC paper. Your genotype should match up with your phenotype of whether you are a strong taster, weak taster, or nontaster.
The Genetics of Bitter Taste
The ability to taste bitter tastes, such as PTC, originally evolved as a mechanism to protect humans from eating poisonous plants and other toxins.
With this discovery, scientists believe that there had to have been some sort of advantage to the PTC nontasters otherwise their gene would've died out due to natural selection. They believe there is some other bitter compound that nontasters of PTC can taste that tasters can't. This would actually lead to heterozygote advantage within the PTC tasting gene.
However, there is still a lot more research to do on the PTC gene, as they have found numerous influences on whether you're a taster or nontaster. Some people can only taste it on certain days, whereas other people have a different phenotype based on what they've ate that day.
Conclusion Question 1
In the first part of the DNA isolation, you discarded the supernatant and kept the cells. However, after processing the sample with Chelex®, you kept the supernatant and discarded the pellet. Tracing the path of your DNA, explain the goal of each processing step.
First, we isolated the DNA within the cell to look at just the sequences. We then used PCR to make millions of copies of our DNA to study. After that, we used a restriction enzyme to cut our DNA sequences based on its specific nucleotide strands and the we used gel electrophoresis to visualize our genotype.
Conclusion Question 2
Explain how the HaeIII enzyme discriminates between the C-G polymorphism in the TAS2R38 gene.
Haelll is a restriction enzyme that searches specifically and only for a GGCC sequence. In non-tasters, a SNP has occurred where this sequence does not occur; therefore, the restriction enzyme, Haelll, can no longer recognize the gene.
Conclusion Question 3
3.)According to the class results, how well does your genotype predict PTC-tasting phenotype? Considering that not everyone who can taste PTC tastes it the same way, what does this tell you about classical dominant/recessive inheritance?
If the lab is done properly, the genotype type will always predict the correct phenotype for the PTC tasting gene. PTC tastes differently for certain people depending whether they are strong tasters with a dominant homozygous trait versus if they have the heterozygous trait making them weak tasters.
Conclusion Question 4
Using what you know about genetics, SNPs, and the PTC gene, explain why it is possible for a person to be a “weak taster.”
The PTC gene has two alleles but that allows for 3 genotypes: TT, Tt, and tt. Those with TT can strongly taste the PTC and those with tt don't taste anything at all. This means that those with Tt will be able to taste PTC, but not as strongly as a person with TT.
Conclusion Question 5
5.)Some studies have shown that PTC “tasters” are less likely to become smokers. Why do you think scientists are seeing this correlation?
The ability to taste bitter substances was originally an evolved trait in humans to keep them from eating harmful toxins. Cigarettes contain many harmful toxins that may perhaps be tasted by only certain people.
Conclusion Question 6
6.)How can the techniques described in this lab be used to test for human disease genes? Would this type of testing work on every disease with a genetic component?
This same lab could be done if the specific gene for the disease is known in order to test humans for disease genes. The same steps including PCR, using a specific restriction enzyme, and gel electrophoresis would be performed to test whether the DNA had the proper sequence of the disease.
Conclusion Question 7
What ethical issues are raised by human DNA typing experiments?
A lot of parents, if they had the money, would prefer to DNA test their own children to test for genetic diseases. However, some do not think this is ethically moral because the children do not have much of a say and might have to worry most of their lives about a disease that they have tested positive for.