Orthologous genes are best defined as genes:
Orthologous genes arise when ancestral gene present in common ancestor diverges as populations speciate, carrying copies into distinct lineages. Despite independent evolution, orthologs often maintain conserved function, synteny and regulatory context, serving as reliable anchors for comparative genomics, phylogenetic reconstruction and functional annotation. Sequence similarity reflects species divergence time rather than duplication. Examples include human and mouse beta-globin or insulin genes. Distinction from paralogs that arise within genome via duplication is crucial for inferring evolu
Ref: Fitch WM, Syst Zool 1970: Defining Orthology as Divergence by Speciation, Evolutionary Concepts