orthologous

Orthologous describes genes in different species that originated from a single ancestral gene through speciation. Orthologous genes often retain similar or conserved functions, making them central to comparative genomics and functional annotation across species.

In contrast, paralogous genes arise by gene duplication within a lineage and may diverge in function. Distinguishing orthologous from paralogous relationships is essential for accurate cross-species function inference.

Orthology is inferred from phylogenetic trees, reciprocal best hits, and conserved synteny, and is curated in databases such as OrthoDB, eggNOG, OMA, and InParanoid.

An example is the human BRCA1 gene and the mouse Brca1 gene, which are orthologous. Limitations include functional divergence after speciation, rapid evolution, and lineage-specific gene loss or expansion.

These relationships are used to transfer functional annotations from model organisms to humans and to illuminate gene family evolution across species.