The frequency of PTC nontasters (tt genotype) is higher than expected if there is a selective advantage to tasting bitter plants or foods in general. PTC is similar to known toxic alkalods that can lead to problems in thyroid metabolism, thus losing the ability to taste these compounds should be a disadvantage. There is evidence that an evolutionary process known as ''balancing selection'' may be responsible, tending to favor heterozygotes ( more fit) over either the strong tasters (TT) or non-tasters (tt). What advantage would heterozygotes have over the homozygotes? ( You will need to do a little research on the internet to come up with a good hypothesis for this).