Taste Buds on the Tongue
Since the early 20th century, it has been determined that some aspects of the sensation of taste are related to genetics. Recent studies have identified the existence of different proteins as being fully responsible for taste. These proteins come in the form of receptors that are found on taste buds – the small structures that exist on the upper surface of the tongue and are integral for the sensation of taste. As saliva begins to break down food, it comes in contact with the taste buds which house taste receptors. The 10,000 taste buds on the average human tongue then send information gathered in these receptors to the brain via cranial nerves.
Each taste experience is caused by a specific type of receptor on the taste bud. Bitterness has been shown to be caused by the receptors TAS2R38 and gustducin, a G protein. Sodium ions are specifically responsible for saltiness. Acidity is detected by hydronium ions creating the sensation of sourness. Sweetness is felt by the presence of sugars, aldehydes and ketones found on the taste buds. Still being fully understood, fermented and aged foods produce the taste sensation known as umami. It is believed that the receptor CD36 is responsible for identifying fatty acids, although research is still being conducted. CaSR was discovered on the tongues of mice in 2008. This is believed to be responsible for identifying calcium.
Above right: Taste buds. (Supplied by Gray’s Anatomy; Public Domain; https://upload.wikimedia.org/wikipedia/commons/5/5a/Gray1018.png)
Types of Taste Receptors
Scientists have determined that the taste receptors for sour and salty are caused by pore-forming proteins called ion channels. These proteins are found on the area that protects the interior of cells known as the plasma membrane. The main purpose of these channels is to regulate the flow of ions on the tongue based on a charge. Ion channels convey ions such as sodium or potassium through the membrane almost like a gatekeeper. Other factors affecting ion channels are temperature or chemical reactions.
G protein-coupled receptors are found only in eukaryotes and are part of the family of proteins known as transmembrane receptors. Basically, these receptors detect molecules outside of the cell and initiate transduction through pathways in the tongue. G protein-coupled receptors are responsible for the taste sensations of bitter, sweet, and umami.
Above left: Closeup of Mouth. (Supplied by Gray’s Anatomy; Public Domain; https://upload.wikimedia.org/wikipedia/commons/6/6e/Gray1014.png)
PTC, or penylthiocarbamide, is a widely-used compound in the study of genetics associated with taste. Depending on the individual, it can either taste very bitter or have nearly no taste whatsoever. The PTC test can be used to help identify the genetic background of subjects. While nearly 70 percent of the human population can taste the compound, certain groups of people have higher or lower rates. For example, geneticists have shown that at least 98 percent of Native Americans taste PTC, far above the average rate.
The phenomenon was first identified by Dr. Arthur Fox in 1931. While working at DuPont, Fox accidentally released a cloud of PTC that was ingested not only by him, but also another colleague. The colleague experienced a bitter taste, while Fox did not.
The ability to taste PTC (or otherwise) is determined by the PTC gene. There are several different alleles (forms) and what determines the intensity of the bitter taste (how strong it is or even if it has no taste at all) are the shapes of the bitter taste receptor proteins that PTC genes code for, and how strongly these bind to PTC.
Above right: PTC. (Supplied by Benjah-bmm27 at Wikimedia Commons; Public Domain; https://upload.wikimedia.org/wikipedia/commons/4/49/Phenylthiourea-from-xtal-3D-balls.png)
So far, geneticists have been unable to identify the exact reason why certain individuals experience stronger sensations in taste than others. These people are called supertasters. Approximately 25 percent of the world’s population are supertasters, most commonly women. It has been shown that these people do have an increased amount of taste buds, however, it does not fully explain the phenomenon. Many scientists believe it may be an evolutionary condition that helped early humans avoid toxic plants that contained alkaloids.
"Taste and Behavior Genetics" Gene Expression (https://www.gnxp.com/MT2/archives/003665.html)
"Genes and Bitter Taste" University of Utah (https://learn.genetics.utah.edu/content/begin/traits/ptc/)