Allopolyploid species form through the fusion of two differentiated genomes and, in the earliest stages of their evolution, essentially all genes in the nucleus are duplicated. Because unique mutations occur in each ancestor prior to allopolyploidization, duplicate genes in these species potentially are not interchangeable, and this could influence their genetic fates. This study explores evolution and expression of a simple duplicated complex--a heterodimer between RAG1 and RAG2 proteins in clawed frogs (Xenopus). Results demonstrate that copies of RAG1 degenerated in different polyploid species in a phylogenetically biased fashion, predominately in only one lineage of closely related paralogs. Surprisingly, as a result of an early deletion of one RAG2 paralog, it appears that in many species RAG1/RAG2 heterodimers are composed of proteins that were encoded by unlinked paralogs. If the tetraploid ancestor of extant species of Xenopus arose through allopolyploidization and if recombination between paralogs was rare, then the genes that encode functional RAG1 and RAG2 proteins in many polyploid species were each ultimately inherited from different diploid progenitors. These observations are consistent with the notion that ancestry can influence the fate of duplicate genes millions of years after duplication, and they uncover a dimension of natural selection in allopolyploid genomes that is distinct from other genetic phenomena associated with polyploidization or segmental duplication.