The ability to taste certain bitter compounds is a straightforward trait that reveals much about human genetics and evolution. This difference in perception is best illustrated by a simple chemical test where individuals are exposed to a specific substance. For some, this substance is intensely bitter, while for others, it is virtually tasteless, representing a clear example of a human polymorphism. This phenomenon shows how a single gene can influence a basic sensory experience.
Phenylthiocarbamide: The Chemical and the Observation
The chemical at the center of this phenomenon is Phenylthiocarbamide, or PTC, a synthetic compound also known as phenylthiourea. PTC is an organosulfur thiourea molecule characterized by the thiourea moiety, which contains a sulfur-nitrogen bond (N-C=S). This structure is responsible for the bitter taste sensation in sensitive individuals.
The discovery of this varied perception was accidental. In 1931, chemist Arthur Fox synthesized crystalline PTC; a colleague complained of a bitter taste, yet Fox tasted nothing. This observation led to the realization that the ability to taste PTC was not universal but depended on an individual’s genetic makeup.
When tested with PTC, people fall into two groups: “tasters” and “non-tasters.” Tasters describe the experience as intensely bitter, even at low concentrations. Non-tasters perceive no taste. Approximately 70% of the human population are classified as tasters, though this percentage varies across different ethnic and geographic groups.
The Genetics of Bitter Perception
The difference between tasters and non-tasters is governed by variations in a single gene, TAS2R38 (Taste Receptor Type 2 Member 38). This gene provides instructions for a bitter taste receptor protein, which is expressed on specialized cells within the taste buds. The TAS2R38 protein is a G protein-coupled receptor (GPCR) that recognizes compounds containing the thiourea structure, such as PTC and PROP (propylthiouracil).
The variation in tasting ability is explained by two major alleles of the TAS2R38 gene. The dominant Taster allele is PAV (Proline-Alanine-Valine), and the recessive Non-Taster allele is AVI (Alanine-Valine-Isoleucine). These two isoforms are defined by changes at three specific positions, called single nucleotide polymorphisms (SNPs).
Individuals inherit one allele from each parent, resulting in three possible genotypes that determine the phenotype. A person with two Taster alleles (PAV/PAV) is a homozygous taster. Someone with two Non-Taster alleles (AVI/AVI) is a homozygous non-taster. Heterozygotes (PAV/AVI) are classified as tasters, but their sensitivity falls between the two homozygous groups. The PAV allele codes for a functional receptor that binds to PTC, while the AVI allele results in a non-functional receptor.
Why PTC Tasting Matters
The ability to taste PTC is strongly correlated with the perception of other naturally occurring bitter compounds. PTC sensitivity predicts a person’s sensitivity to PROP (6-n-propylthiouracil), a chemically related compound. This sensitivity also extends to glucosinolates, the bitter compounds found in cruciferous vegetables such as broccoli, cabbage, and kale.
The variation in bitter taste perception influences dietary choices, particularly the intake of these vegetables. Highly sensitive tasters (PAV/PAV) may find these vegetables too bitter and consume less of them. Non-tasters (AVI/AVI) are not deterred by the bitterness and may consume them more readily. This difference in diet selection has implications for nutrition and susceptibility to diet-related health conditions.
From an evolutionary standpoint, this taste variation likely conferred a selective advantage to early humans. Many toxic plants produce bitter compounds, and the ability to taste these substances helped ancestors avoid accidental poisoning. The persistence of both taster and non-taster alleles suggests balancing selection has maintained diversity. The study of the TAS2R38 gene serves as a classic marker for studying human population genetics.