In order to have 100% heterozygotes, we must have one parent with the traita, but homozygote (TT) and other without the trait, that is recessive homozyogte. In that case, each parent could only produce T or t alleles, therefore the offspring only have the chance to be Tt. So one parent must be TT and the other must be non taster (as this reflects to be recessive homozygote).