Much of the variation in tasting of PTC is associated with polymorphism at the TAS2R38 taste receptor gene.  In humans, there are three SNPs ( single nucleotide polymorphisms ) along the gene that may render its proteins unresponsive.  There is conflicting evidence as to whether the inheritance of this trait is dominant or incompletely dominant .  Any person with a single functional copy of this gene can make the protein and is sensitive to PTC. [ citation needed ] Some studies have shown that homozygous tasters experience a more intense bitterness than people that are heterozygous ; other studies have indicated that another gene may determine taste sensitivity.