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To the Bitter End

A Case Study Examining the Genetics of PTC Sensitivity


R. Deborah Overath
Department of Life Sciences
Texas A&M University – Corpus Christi


This interrupted case dramatizes the discovery of a Mendelian trait in humans, namely the variation in the ability to taste the chemical phenylthiocarbamide (PTC). By examining data and questions related to this trait, students will draw connections between Mendel's principles of inheritance and variation at the DNA level by learning about the phenotypic differences due to the single nucleotide polymorphisms (SNPs) in a major gene influencing the ability to taste PTC. Students will review Mendelian inheritance, human pedigrees, genetic markers such as single nucleotide polymorphisms, and gene mutations. If PTC paper is used and student phenotypes collected, this case can be expanded to include a population genetics component on the Hardy-Weinberg Model. The case was developed for a sophomore-level course in genetics.

  • Explain how Mendelian traits are inherited and studied in humans.
  • Explain the role of DNA sequences and mutations in determining protein structure and function.
  • Explain how DNA mutations that compromise protein function may be inherited as a recessive allele.
  • Explain the connection between Mendelian and molecular genetics by generalizing the specific facts learned in this case to other examples.
Keywords: Human genetics; DNA variation; Mendelian inheritance; phenylthiocarbamide; PTC; PTC tasting; bitter taste sensitivity; genotype; phenotype; pedigree analysis; single nucleotide polymorphisms; SNP; SNPs; Hardy-Weinberg model
Topical Area: History of science, Scientific method
Educational Level: High school, Undergraduate lower division
Formats: PDF
Type/Method: Analysis (Issues), Discussion, Interrupted
Language: English
Subject Headings: Genetics / Heredity   Biology (General)   Science (General)  
Date Posted: 7/26/2011
Date Modified: N/A
Copyright: Copyright held by the National Center for Case Study Teaching in Science, University at Buffalo, State University of New York. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.

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The following video(s) are recommended for use in association with this case study.

  Genetics of Bitter Taste Perception hhmi/
In this 50-minute lecture, Dr. Michael Campbell discusses how humans perceive the taste of the chemical PTC. With Dr. Sarah Tishkoff, he fields questions about the evolution of taste perception and scientific career choices. Produced by HHMI BioInteractive.

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