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; http://upload.wikimedia.org/wikipedia/commons/4/49/Phenylthiourea-from-xtal-3D-balls.png)