Question

5.) Why might the two alleles, taster and non-taster, persist in human populations over many generations? (Hint, what is the advantage of this diversity?)

Answer 1

Answer -

The ability to taste the bitter compound phenylthiocarbamide (PTC) and related chemicals is bimodal, and all human populations tested to date contain some people who can and some people who cannot taste PTC. Why this trait has been maintained in the population is uncertain but this polymorphism may influence food selection, nutritional status or thyroid metabolism.

The ability to taste the compound phenylthiocarbamide (PTC) varies dramatically among humans. Approximately 75% of people find PTC intensely bitter, whereas 25% do not detect it.

One possibility is that PTC heterozygotes gain a fitness advantage through the perception and avoidance of a larger repertoire of bitter toxins than homozygotes. the PTC-sensing phenotype may confer certain benefits. It has been suggested, for example, that PTC tasters might enjoy an evolutionary advantage by avoiding bitter-tasting toxic compounds, and also that a genetically encoded ability to taste bitter compounds could be protective against nicotine and alcohol consumption that might otherwise lead to addiction.

This phenotype is of genetic, epidemiologic, and evolutionary interest because the ability to taste PTC is correlated with the ability to taste other bitter substances, many of which are toxic. Thus, variation in PTC perception may reflect variation in dietary preferences throughout human history and could correlate with susceptibility to diet-related diseases in modern populations.

some researches :

• Although PTC itself has not been found in nature, the ability to taste PTC is correlated strongly with the ability to taste other naturally occurring bitter substances, many of which are toxic (Harris and Kalmus 1949; Barnicot et al. 1951; Tepper 1998).

• Furthermore, variation in PTC taste sensitivity has been correlated with dietary preferences that may have significant health effects (Bartoshuk et al. 1994). For example, PTC is similar in structure to isothiocyanates and goitrin, both of which are bitter substances found in cruciferous vegetables like cabbage and broccoli (Tepper 1998).

• Variable aversions to these compounds have been implicated in the variable rates of thyroid-deficiency disease in PTC tasters and nontasters, with nontasters being more susceptible (Drewnowski and Rock 1995).

• R. A. Fisher’s hypothesis that balancing natural selection has maintained taster and nontaster alleles appears to hold true in humans (Fisher et al. 1939).

The correlation of PTC perception with the perception of other toxic bitter substances and the association of the inability to perceive PTC with disease susceptibility combine to suggest that natural selection may have been an important factor in the evolution of this trait. R. A. Fisher hypothesized that the pervasive phenotypic variation in PTC perception is due to balancing natural selection, which may have favored heterozygotes.

Hence it can be suggested that due to above advantages both the alleles have persisted throughout so many generations of human populations.