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Abstract
Orthologs and paralogs are two fundamentally different types of homologous genes that
evolved, respectively, by vertical descent from a single ancestral gene and by duplication.
Orthology and paralogy are key concepts of evolutionary genomics. A clear distinction
between orthologs and paralogs is critical for the construction of a robust evolutionary
classification of genes and reliable functional annotation of newly sequenced genomes.
Genome comparisons show that orthologous relationships with genes from taxonomically
distant species can be established for the majority of the genes from each sequenced
genome. This review examines in depth the definitions and subtypes of orthologs and
paralogs, outlines the principal methodological approaches employed for identification
of orthology and paralogy, and considers evolutionary and functional implications
of these concepts.