The ability to taste the bitter chemical phenylthiocarbamide (PTC) is an example of dimorphism, a type of polymorphism, with the coexistence of two phenotypes of a character. Most human population are dimorphic, can either detect PTC as a foul, bitter taste or cannot taste it at all. The pedigree in Figure (a) is an example of how two tasters sometimes produce nontaster children.1 According to this pedigree we can conclude that the allele that confers the ability to taste is dominant and that the allele for non tasting is recessive.1
Figure (a): Pedigree for the ability to taste the chemical PTC.
Variation in PTC sensitivity was first discovered in a lab incident in the early 1930s by ...view middle of the document...
One of these two proteins, as revealed by subsequent work, acts as a receptor for bitter ligands and is one of many such receptors found in mammals (Adler et al. 2000; Chandrashekar et al. 2000; Shi et al. 2003). The TAS2R gene family encodes these small proteins which are less than 400 amino acids in length. The TAS2R receptors expressed in the apical microvilli of bitter taste receptors, are exposed to the oral cavity through a taste pore (Hoon et al. 1999), where in such position can come into contact with the many compounds that enter the mouth, as shown in the illustrations below.3 Each of these compounds represent a potential ligand, capable of binding to a receptor and subsequently stimulating a bitter taste perception.2
The discovery of the bitter taste receptors led to a series of studies of the inheritance of PTC sensitivity as in 1999, Reed et al. identified significant linkage near a bitter taste receptor (TAS2R01) on chromosome 5. The definitive results however were obtained in 2003 by Kim et al. in association analysis and by Drayna et al. in a linkage analysis of the Utah CEPH Pedigrees. These analyses revealed that variation at the TAS2R38 locus is responsible for 50 to 80% of phenotypic variance in PTC sensitivity. Most of this variance is accounted for by the presence of two common haplotypes, a “taster” haplotype and a “non-taster” haplotype.2 The behavior of proteins based on lipophilic and lipophobic stretches of amino acids can determine the structure of the bitter taste receptors. The ability of the TAS2R38 to activate the G-protein can determine the variation in bitter tasting. This affects the interaction between the ligand and receptor.
Figure (b): The darker the shade of the color, the more likely that population is to taste the PTC and the lighter the shade of the color, the less likely that population is to taste the PTC.2
Genetically mediated sensitivity to the bitter taste of PTC is a classic inherited trait in humans and has been the subject of genetic studies for years. This trait has also been shown to be linked to preferences for sugar and fat levels in food, fruit and vegetables. It correlates to their patterns of food rejection and acceptance and even alcoholism.5 Subsequently that may have important implications for human health, body weight and risk of chronic diseases.6 For instance, consumption of cruciferous vegetables that are among the major dietary sources of potentially chemoprotective agents is reported to alter cancer risk.7 Despite the correlation between TAS2R38 genotype and dietary preferences that have been found among children, there was no association reported among older adults. This suggests that environmental factors are more influential than genetic influences in food preferences among the elderly. Also, the elderly may be more inclined to...