ortoloogia

Ortology, or orthology, is a concept in comparative genomics that describes the relationship between genes in different species that originated by speciation from a single ancestral gene. Orthologous genes are expected to retain similar functions across species, whereas paralogous genes arise by gene duplication within a lineage and may diverge in function. The concept underpins cross-species functional annotation, evolutionary inference, and the reconstruction of gene-family histories.

Orthologs can be identified by phylogenetic methods that reconcile gene trees with species trees, or by sequence-based approaches such as reciprocal best hits and clustering into orthogroups. Synteny and genomic context can improve accuracy in some cases. Popular resources and databases include OrthoDB, eggNOG, InParanoid, and OMA. Limitations of orthology inference include incomplete lineage sorting, gene loss, convergent evolution, and events like horizontal gene transfer, which can blur distinctions between orthologs and paralogs.

Applications of orthology include transferring functional annotations between species, guiding model organism research, and studying genome evolution. A well-known example is Pax6, which has orthologs across vertebrates and, in Drosophila, the eyeless gene is considered an ortholog; these genes control eye development in their respective lineages. By comparing orthologs, researchers can infer conserved functions and identify lineage-specific innovations.