Ortholog
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Classically, orthologs are genes in different species which evolved from a common ancestral gene. Due to their separation following a speciation event, orthologs may diverge, but are similar in sequence or structure, and usually perform similar functions. Orthology, like paralogy, is a type of homology.
A second definition of ortholog has arisen to mean any two genes in two different species with very similar functions. This differs from the original definition in that there is no statement about evolutionary relation, or similarity in sequence or structure.
Orthologous sequences provide useful information in taxonomic classification studies of organisms. The pattern of genetic divergence can be used to trace the relatedness of organisms. Two organisms that are very closely related can display very similar DNA sequences between two orthologs. Conversely, an organism that is further removed evolutionarily from another organism can display a greater divergence in the sequence of the orthologs being studied.
[edit] Ortholog vs. Paralog
The term "ortholog" was coined in 1970 to describe two similar genes in two different species that originated from a common ancestor. Despite this common origin, the genes need not necessarily have the same function. Orthologs arise primarily from speciation.
Paralogs are similar genes within a single species that diverged by gene duplication. Paralogous sequences provide useful insight to the way genomes evolve.
These classifications are subtypes of homology (biology): two homologous genes can be orthologous or paralogous. Homology in this context is measured by comparing the DNA or protein sequences of the similar genes.
