Templates in C++ provide a sophisticated mechanism for writing generic, polymorphic code (i.e. parametric polymorphism). Benzene metabolism involves enzymes coded for by polymorphic genes. Some species are polymorphic for different chromosome structural forms. Genes undergoing positive selection have an excess of divergent sites relative to polymorphic sites. For instance, type inference with polymorphic recursion is known to be undecidable. Groups of people around the world have different frequencies of polymorphic genes. There have been many hypothesis for the genetic cause of this polymorphic colouration. An adenosine present at the polymorphic site represents the third position in a stop codon. Such languages may also enable functions to be parametric polymorphic. In such a setting, making arrays invariant rules out useful polymorphic programs. This is achieved by adding constraints to type variables in parametrically polymorphic types. The chromosome addition of Y Alu polymorphic element is only displayed in Japanese American men. PGH does this by using polymorphic markers that are better suited at recognizing translocations. Some researchers treated it as a highly polymorphic species with several subspecies. Also seen in catecholaminergic polymorphic ventricular tachycardia (CPVT).