Question

3. Can you find any pattern related to PTC tasters vs. nontasters? You will have to “interview” other members of the class to get information. (For example, do tasters appear to be predominantly male or female? Is taste perception of PTC correlated with PROP or SB? Do PTC tasters like bitter foods more less? Etc.)

4. Would people who are strong tasters of PTC and/or PROP be likely to find beer particularly bitter or not? Explain your conclusion.

5. A big question that early geneticists attempted to answer is, what evolutionary forces might be affecting PTC taste variation. PTC is a fully synthetic substance and does not occur in nature. However, the perception of bitter taste may well have adaptive value as many plant toxins and medicinal active chemicals are bitter. In fact, chimps were found to have nearly the same frequency of taster vs. nontaster alleles as humans. In 1950, it was discovered that taste perception of l-5-vinyl-2-thio-oxazolidone, a toxic compound in cabbage that can cause goiters, corresponds exactly to PTC taste sensitivity. If the ability to taste PTC corresponds to the ability to taste certain naturally-occurring chemicals, then the PTC taster allele could have adaptive value. Do the population genetics data support this speculation?

6. Speculate on how selection might maintain the PTC alleles at their current frequencies.

Answer 1

1. Both, PTC tasters and non-tasters shows presence of Testing allele protein encoded by the TAS2R38 gene. In case of non tasters, the receptors are either absent or partially abnormal due to translational defects.

• In a research carried out in Kashmir (India), the Chi square value for females was 4.56 (p=0.471) as compared to males who had Chi square value of 5.254 (p=0.385).
• PROP (6-n-Propylthiouracil) is an analog having similar characters as those of PTC and has the same binding potential considered in the form of ligand binding.
• Addressing the next part and Q.4 ​​​​​​, bitter taste sensitivity can vary. Genetics plays an important role in determining the fate of the receptor. Bitter taste is a stimulation to the gustatory cells to not consume the product. However, environmental factors play an important role in shaping human behavior with respect to stimulation of PTC and beer. (Stress mediated conditions can prompt the person to consume alcohol irrespective of being bitter. Continuous exposure may project a more advanced affection towards alcohol).
• Research carried out on ethanol consumers with variations in TAS2R16 and TAS2R38 shows more inclined nature towards ethanol. It may vary in taste from sweet to bitter showing gradual variations.

Q.5. There are natural compounds like L-5-vinyl-2-thio-oxazolidone found in cabbage which are also specified with respect to sensitivity in Tasters and Non-tasters. RA Fisher has elaborated about haplotypes and variability of PTC with respect to natural selection. A recent research carried out on humans and chimpanzees conclude alteration of 5 nucleotides (humans) and 3 nucleotides (chimpanzee) which impacted in minor variations in amino acid attachment. This is non-synonymous substitution which favors positive natural selection without impacting the prior role of PTC related receptor gene.

Alleles like A49P, A262V and V2961 are major highlights.

Certain studies shows variations in terms of consumption of compounds similar to PTC. Hence it cannot be thoroughly considered as an adaptation but can act as an external sensing apparatus on the basis of past encounters with a particular PTC-like toxin. ( If someone encounters a bitter compound and had undesired stimulation, he may end up not eating it again. It also depends on mood and certain hormonal secretions. These may facilitate either acceptance or disapproval of such a compound. But prior information about PTC consumption also should be considered. If someone knows he is eating a toxin, he wouldn't consume it. But if presented unknowingly, the fate may be either the person would consume and keep a memory of the taste that has been stimulated which can either favor it's acceptance or not. So, adapting nature should not be considered in such cases.)