Evolutionary preservation of redundant duplicated genes

Gene duplication events produce both perfect and imperfect copies of genes. Perfect copies are said to be functionally redundant when knockout of one gene produces no 'scoreable: phenotypic effects. Preserving identical, dupl icate copies of genes is problematic as all copies are prone to accumulate neutral mutations as pseudogenes, or more rarely, evolve into neu genes wit h novel functions. We summarise theoretical treatments for the invasion and subsequent evolutionary modification of functionally redundant genes, we t hen consider the preservation of functionally identical copies of a gene ov er evolutionary time. We present several models for conserving redundancy: asymmetric mutation asymmetric efficacy, pleiotropy, developmental bufferin g, allelic competition and regulatory asymmetries. In all cases, some form of symmetry breaking is required to maintain functional redundancy indefini tely.