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PTC tasting: The myth To some people, small amounts of the compounds phenylthiocarbamide (PTC) or propylthiouracil (PROP) taste very bitter; other people do not taste these compounds. The myth is that there are only two kinds of people, tasters and non-tasters, and that the trait is controlled by a single gene, with the allele for tasting dominant over the allele for non-tasting. The reality PTC tasting as a character Fox (1932) was working in a lab with phenylthiocarbamide (PTC) when a colleague complained about the bitter taste of the chemical dust Fox was spreading around. Fox insisted that it was tasteless; to settle the disagreement, he had other colleagues taste the PTC and discovered that it had a strong bitter taste for some people, while others found it tasteless. This led to a large body of research on PTC tasting; Guo and Reed (2001) review the subject and cite 392 references, and there has been more work done since then. For most of the myths in this series, I had to dig deep to find every obscure scrap of information, but for PTC, I am just skimming the surface. The reviews by Guo and Reed (2001), Kim and Drayna (2004) and Wooding (2006) are good places to start if you'd like more information. Testing methods Some of the early studies put PTC crystals directly on the tongue, while others used solutions of PTC or paper soaked in PTC and then dried. However, some people would be classified as tasters with one technique and non-tasters with a different technique (Hartmann 1939, Lawless 1980). The most common method for measuring the ability to taste PTC involves finding the weakest concentration of PTC that tastes different from plain water (Blakeslee 1932). The technique of Harris and Kalmus (1950) for threshold measurement has been widely used, sometimes with small modifications. The subject is given a two-fold dilution series of PTC, starting with the weakest concentration and going up until they say they can taste it. The subject is then asked to sip four PTC solutions of that concentration and four plain waters, and identify which are PTC. If they get it correct, the next weaker solution is tried; if they get it incorrect, the next stronger solution is tried. The weakest PTC solution that the subject can correctly identify is the threshold. Harris and Kalmus (1950) found that the distribution of PTC tasting thresholds was bimodally distributed, but there were some intermediate individuals. Other studies have found similar results, a bimodal distribution with some intermediate individuals (Blakeslee 1932, Salmon and Blakeslee 1935, Falconer 1947, Olson et al. 1989, Whissell-Buechy et al. 1990, Guo et al. 1998, Drayna et al. 2003). A different way to measure PTC tasting is to give each subject an intermediate concentration of PTC solution, then ask them to rate it on a numeric scale, such as 0 (no taste) to 7 (very strong taste) (Lawless 1980). This category rating method is much quicker than the threshold detection method of Harris and Kalmus (1950) and exposes the subjects to much less PTC. It may also do a better job of separating people into two distinct categories, taster and non-taster (Lawless 1980), but it has not been used very often. In classrooms, the usual way to test PTC tasting is by having students taste a piece of paper that has been soaked in PTC. Some individuals who are classified as tasters using paper are non-tasters using a threshold test, and vice versa (Hartmann 1939, Lawless 1980). Khataan et al. (2009) asked subjects to taste a piece of paper containing 3 µg of PTC and rate it from 1 (not at all bitter) to 9 (extremely bitter). The variation among 911 subjects was not at all bimodal. Conclusion PTC tasting is largely determined by a single gene, TAS2R8, with two common alleles, and the allele for tasting is mostly dominant over the allele for non-tasting. However, both classical family and twin studies, and modern molecular genotyping, show that there are other genes or environmental factors that influence PTC tasting. As a result, there is a continuous range of PTC tasting, not absolute separation into tasters and non-tasters. PTC tasting would be a fascinating subject for an advanced genetics class, but it does not fit the one-gene, two-allele myth well enough to be used to demonstrate simple Mendelian genetics. #NikolaysGeneticsLessons #supertaster #tongue #tasteBud #papillae #bitter #sweet #sour #salty #umami #PTC #prop #paper #gene #Dominant #allele #tasteReceptor #sciShow #quickQuestion #hankGreen #science #PTCTestingPaper #PTCTestingStrips #DNA #electrophoresis #biotechnology #scienceExperiment #highSchoolScience #biology #biologyExperiment #biologyTeacher #Education #scienceEducation #biologyEducation #PolymeraseChainReaction #gelElectrophoresis #geneticsExperiment
