Genetic linkage acting through crossing-over between X and X chromosomes, X and Y chromosomes, and autosomal gene recombination are the most important sources of color pattern polymorphisms in animals. Variability in male color patterns and fin morphologies in the guppy, Poecilia reticulata, a livebearing fish is an example of extreme pattern polymorphism. We explored the possibility that crossing-over between Y chromosomes can also contribute to the high degree of pattern polymorphism in guppies because YY individuals are easily induced in the laboratory. However, note that YY individuals are also produced in natural populations. Our results indicated that YY crossing-over was another important source of phenotypic variability-probably because recombination may be possible over the entire length of Y chromosomes, and at very high frequencies due to high degrees of homology. Thus, crossing-over between Y chromosomes is yet another mechanism that can contribute to extreme pattern polymorphism in the guppy, a popular aquarium and important research model species.