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 usually have similarity at the sequence and structure levels; furthermore, orthologs usually have 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 function. This differs from the original definition in that there is no statement about origin or sequence / structural similarity.
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 gene sequences between two orthologs. Conversely, an organism that is further removed evolutionarily from another organism can display a greater variation in the sequence of the two orthologs being studied.
[edit] Orthology vs. Paralogy
Orthology or paralogy of two genes is determined via comparison of the two DNA sequences (one from each gene). These classifications are both subsets of homology (i.e. if two genes are orthologous they are also homologous; if they are paralogous, they are also homologous).
The term "ortholog" was coined in 1970 to describe two 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 genes within a single species that diverged by gene duplication. Paralogous sequences provide useful insight to the way genomes evolve. The duplication of genes has emerged as an important aspect of molecular innovation and the evolution of developmental pathways.
[edit] See also
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