Unique gene representation (path representation in the literature) is a type of enumerated chromosome.  It may be used to resolve problems such as the Traveling Salesman Problem.  Each city in the problem has its unique ordinal number.  The tour is represented as a sequence of numbers of cities to be visited.  Path representation has one significant feature:  there are no duplicate allele values in the chromosome.  Partially-Matched Crossover is used to reproduce individuals with enumerated chromosomes in the unique gene representation.