The frequency of the PTC nontasters (tt genotype) is higher than expected when there’s the selective advantage to tasting bitter plants or foods in general. PTC is similar to recognized toxic alkalods which can lead to problems in thyroid metabolism, thus losing the capability to taste such compounds should be the disadvantage. There is evidence that the evolutionary procedure known as ''balancing selection'' might be responsible, tending to favour heterozygotes ( more fit) over either the strong tasters (TT) or non-tasters (tt). What advantage would heterozygotes have over homozygotes? (You will require doing a little research on the internet to come up with the good hypothesis for this).