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homologue vs orthologue?

Simon Andrews simon.andrews at bbsrc.ac.uk
Tue Jan 9 09:39:09 EST 2001

Mark Fry wrote:
> Can somebody please tell me the difference between homologous genes,
> orthologous genes and paralogous genes.

Homologous genes have shared a common evolutionary ancestor.  Note that 
homology and similarity are not the same thing - genes which are
homologous may have high or low similarity, but the presence of
similarity doesn't necessarily make two sequences homologues (could be
convergent evolution).

Paralogous and orthologous genes are subdivisions of homologous genes. 
All homologs are either orthologs or paralogs.

Orthologs arise because of speciation events.   Thus if a rat and a
human sequence have simply diverged since the last common ancestor of
rats and humans, then they are orthologs.

Paralogs arise because of gene duplication events.  Thus there may be an
alpha and beta form of a human protein which have arisen through gene
duplication, and then diverged.  These two genes are paralogs.  Also
note that if this duplication occurred before the last common human-rat
ancestor then alpha-human is a paralog to beta-rat, but an ortholog to

If all this isn't completely clear then take a look at the slide at;


which may give you a better feel for the differences.

Hope this helps


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