Ortholog
Orthologs are genes in different species that originated from a single gene in the last common ancestor of those species through a speciation event. They typically retain equivalent functions across species, but functional divergence can occur. Orthology is contrasted with paralogy, where genes are related by duplication within a genome, and xenology, where homology arises from horizontal gene transfer. The term emphasizes a genealogical relationship across species rather than a property of a single gene.
The identification of orthologs relies on phylogenetic analyses that reconcile gene trees with species trees to
Orthology yields several relationship types. There are 1-to-1 orthologs (one gene per species), many-to-many orthologs after
Applications and limitations: Orthology is central to comparative genomics, enabling transfer of functional information and reconstruction