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PTC tasting is a classic genetic marker in human population genetics investigations.
In 1931 Arthur Fox, a chemist at DuPont, in Wilmington, Delaware, synthesized phenylthiocarbamide (PTC). Some researchers reported a bitter taste when entering his laboratory, while others, including Fox himself, experienced no such sensation. [1] Further study of this phenomenon by L.H. Snyder in 1931 led to the conclusion that the inability to taste PTC is a recessive trait. [2] In 1932, Albert Blakeslee conducted a large-scale study involving the inheritance of PTC tasting within families that concluded that PTC tasting sensitivity is very likely a complex Mendelian trait whose variance is overwhelmingly dependent on a single gene locus; however, it is likely that a few other genes have a smaller effect as well. [3]
In 1939 Fisher et al. found that the genetic frequency of PTC tasting was the same in chimps and humans. This similarity suggests that whatever gene controls for PTC tasting must have some sort of a selective advantage in order to have either evolved and been maintained for millions of years since before humans and chimps diverged into separate species, or to have evolved in two separate events after species divergence. [4] Since finding out that PTC tasting had an apparent naturally selective advantage, scientists began to hypothesize that this advantage was that the ability to taste natural chemicals similar to PTC helped human ancestors stay away from some toxic plants. Substances that resemble PTC today are in some vegetables from the cabbage family (Brassicaceae), such as broccoli and Brussels sprouts. [5] [ circular reference ]
In 1950, William Boyd found evidence that the same gene that controls for PTC tasting also controls for the "tasting" of a different compound that acts as an antithyroid drug similar to that found in cabbage plants. [6] Despite all of this compelling evidence for PTC "tasters" to have a selective advantage over "non-tasters," there was no explanation for the consistent proportions of "non-taster" in human and chimp populations until scientists discovered that subjects with a "PTC non-taster" phenotype were able to taste a different bitter compound that "PTC tasters" could not. This compound was juice from an antidesma plant and notably, this study found a perfectly inverse relationship between PTC and antidesma tasting. [7] These results were replicated in 2005 by another team of researchers who found the same inverse relationship between PTC and antidesma tasting [8] However, in 2018, Davide Risso and colleagues expanded upon this relationship when they discovered a notable difference from previous studies. What they found was that there appeared to be a small population of people who could taste both "PTC" and antidesma as bitter. [9] This finding could suggest a possible advantage for heterozygotes given their ability to taste both bitter compounds.
In 1999, Mark Hoon and a team of researchers discovered a gene family that codes for taste receptors, specifically for the "bitter" flavor which they called the TAS2R gene family. The locus of the gene (or genes) that control for PTC tasting is hypothesized to be a part of this TAS2R gene family. In 2003, Dennis Drayna and his colleagues at the National Institutes of Health (NIH), [10] as well as a team of researchers led by Un-kyung Kim, [11] discovered that a variation at the TAS2R38 gene locus is responsible for an overwhelming majority of the variance in PTC tasting sensitivity (50-80%).
It has been suggested that taste and smell receptors are controlled by TAS2R38, with a small intron gene of about 1000 nucleotides. It is a member of the family of G protein-coupled or 7 trans membrane cross receptors. The binding of a ligand to the extracellular region of the receptor sets an action potential that sends an impulse to the sensory cortex of the brain, where it is interpreted as a bitter taste.
This allows an experimental test for SNP at position 145 that has the highest correlation to the sample 3 polymorphisms. The test isolates DNA from cheek cells by a simple salt mouthwash and amplification of a region of the gene TAS2R38. The amplified fragment (amplicon) is incubated with the restriction enzyme HaeIII, comprising the SNP in their recognition sequence GGCC. HaeIII cuts the taster allele (having the sequence GGCC); this generates a length polymorphism, and the 2 alleles can be easily separated in an agarose gel.
Virtually all non-tasters (dd) cannot taste PTC, while homozygous tasters (TT) occasionally report an inability or weak ability to taste the chemical. The heterozygous genotype (Tt) has the "leakiest" phenotype as reduced or absent tasting ability is relatively common. This is formally called a heterozygous effect.
In 1949, Harris and Kalmus developed a method for differentiation of bimodal threshold stimuli for tasting PTC. They proposed a series of 13 solutions of these substances with serial water by halves from the initial concentration of 0.13%, so that the solution in the final test contained only a few molecules of this substance. Pure water was used as the fourteenth test liquid to provide a control. Differentiation between the two phenotypes of "tasters" and "non-tasters" occurred with the fifth solution. Then assuming that the conditional dimorphism controlled by two allele of the corresponding gene locus, the allele which controls the absence of sensitivity to taste PTC is recessive homozygote. [12] [13]
| Solution | PTC (%) |
| 1 | 0.13 |
| 2 | 0.065 |
| 3 | 0.0325 |
| 4 | 0.01625 |
| 5 | 0.008125 |
| 6 | 0.0040625 |
| 7 | 0.00203125 |
| 8 | 0.001015625 |
| 9 | 0.0005078125 |
| 10 | 0.00025390625 |
| 11 | 0.000126953125 |
| 12 | 0.0000634765625 |
| 13 | 0.00003173828125 |
| 14 | Boiled tap water was used both for making up the solutions and for controls. |
Although the view of the genetics of individual sensitivity to taste PTC changed, practically all the current data on the PTC taste (in)ability established certain of these substances originate from research by Harris and Kalmus, and such investigations are still taken. This is probably because it is not suggested a better method for mass population genetics projects.
| Location | # of Participants | Non-taster % | References |
|---|---|---|---|
| Bosnia and Herzegovina | 7,362 | 32.02 | Hadžiselimović et al. (1982) [14] |
| Croatia | 200 | 27.5 | Grünwald, Pfeifer (1962) |
| Czech Republic | 785 | 32.7 | Kubičkova, Dvořaková (1968) |
| Denmark | 251 | 32.7 | Harrison et al. (1977) [15] |
| England | 441 | 31.5 | Harrison et al. (1977) [15] |
| Hungary | 436 | 32.2 | Forai, Bankovi (1967) |
| Italy | 1,031 | 29.19 | Floris et al. (1976) |
| Montenegro | 256 | 28.20 | Hadžiselimović et al. (1982) [14] |
| Užice, Serbia | 1,129 | 16.65 | Hadžiselimović et al. (1982) [14] |
| Voivodina, Serbia | 600 | 26.3 | Božić, Gavrilović (1973) |
| Russia | 486 | 36.6 | Boyd (1950) |
| Slovenia | 126 | 37.2 | Brodar (1970) |
| Spain | 203 | 25.5 | Harrison et al. (1977) [15] |
In genetics and bioinformatics, a single-nucleotide polymorphism is a germline substitution of a single nucleotide at a specific position in the genome that is present in a sufficiently large fraction of considered population.
Phenylthiocarbamide (PTC), also known as phenylthiourea (PTU), is an organosulfur thiourea containing a phenyl ring.
Brassica oleracea is a plant species from family Brassicaceae that includes many common cultivars used as vegetables, such as cabbage, broccoli, cauliflower, kale, Brussels sprouts, collard greens, Savoy cabbage, kohlrabi, and gai lan.
A supertaster is a person whose sense of taste is of far greater intensity than the average person, having an elevated taste response.
Aftertaste is the taste intensity of a food or beverage that is perceived immediately after that food or beverage is removed from the mouth. The aftertastes of different foods and beverages can vary by intensity and over time, but the unifying feature of aftertaste is that it is perceived after a food or beverage is either swallowed or spat out. The neurobiological mechanisms of taste signal transduction from the taste receptors in the mouth to the brain have not yet been fully understood. However, the primary taste processing area located in the insula has been observed to be involved in aftertaste perception.
TAS2R16 is a bitter taste receptor and one of the 25 TAS2Rs. TAS2Rs are receptors that belong to the G-protein-coupled receptors (GPCRs) family. These receptors detect various bitter substances found in nature as agonists, and get stimulated. TAS2R16 receptor is mainly expressed within taste buds present on the surface of the tongue and palate epithelium. TAS2R16 is activated by bitter β-glucopyranosides
Cruciferous vegetables are vegetables of the family Brassicaceae with many genera, species, and cultivars being raised for food production such as cauliflower, cabbage, kale, garden cress, bok choy, broccoli, Brussels sprouts, mustard plant and similar green leaf vegetables. The family takes its alternative name from the shape of their flowers, whose four petals resemble a cross.
A taste receptor or tastant is a type of cellular receptor which facilitates the sensation of taste. When food or other substances enter the mouth, molecules interact with saliva and are bound to taste receptors in the oral cavity and other locations. Molecules which give a sensation of taste are considered "sapid".
Taste receptor 2 member 38 is a protein that in humans is encoded by the TAS2R38 gene. TAS2R38 is a bitter taste receptor; varying genotypes of TAS2R38 influence the ability to taste both 6-n-propylthiouracil (PROP) and phenylthiocarbamide (PTC). Though it has often been proposed that varying taste receptor genotypes could influence tasting ability, TAS2R38 is one of the few taste receptors shown to have this function.
Taste receptor type 2 member 10 is a protein that in humans is encoded by the TAS2R10 gene. The protein is responsible for bitter taste recognition in mammals. It serves as a defense mechanism to prevent consumption of toxic substances which often have a characteristic bitter taste.
Taste receptor type 2 member 14 is a protein that in humans is encoded by the TAS2R14 gene.
Taste receptor type 2 member 43 is a protein that in humans is encoded by the TAS2R43 gene.
Taste receptors for bitter substances (T2Rs/TAS2Rs) belong to the family of G-protein coupled receptors and are related to class A-like GPCRs. There are 25 known T2Rs in humans responsible for bitter taste perception.
Taste receptor type 2 member 20 is a protein that in humans is encoded by the TAS2R20 gene.
Taste receptor type 2 member 50 is a protein that in humans is encoded by the TAS2R50 gene.
The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception stimulated when a substance in the mouth reacts chemically with taste receptor cells located on taste buds in the oral cavity, mostly on the tongue. Taste, along with the sense of smell and trigeminal nerve stimulation, determines flavors of food and other substances. Humans have taste receptors on taste buds and other areas, including the upper surface of the tongue and the epiglottis. The gustatory cortex is responsible for the perception of taste.
Laurence Hasbrouck Snyder was a pioneer in human genetics and president of the University of Hawaii.
The evolution of bitter taste receptors has been one of the most dynamic evolutionary adaptations to arise in multiple species. This phenomenon has been widely studied in the field of evolutionary biology because of its role in the identification of toxins often found on the leaves of inedible plants. A palate more sensitive to these bitter tastes would, theoretically, have an advantage over members of the population less sensitive to these poisonous substances because they would be much less likely to ingest toxic plants. Bitter-taste genes have been found in a variety of species, and the same genes have been well characterized in several common laboratory animals such as primates and mice, as well as in humans. The primary gene responsible for encoding this ability in humans is the TAS2R gene family which contains 25 functional loci as well as 11 pseudogenes. The development of this gene has been well characterized, with proof that the ability evolved before the human migration out of Africa. The gene continues to evolve in the present day.
Dennis T. Drayna is an American human geneticist known for his contributions to stuttering, human haemochromatosis, pitch, and taste. He is currently the Section Chief of Genetics of Communication Disorders at the U.S. National Institute for Deafness and Other Communication Disorders.
Sarah Anne Tishkoff is an American geneticist and the David and Lyn Silfen Professor in the Department of Genetics and Biology at the University of Pennsylvania. She also serves as a director for the American Society of Human Genetics and is an associate editor at PLOS Genetics, G3, and Genome Research. She is also a member of the scientific advisory board at the David and Lucile Packard Foundation.
