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homologue vs orthologue?

Emir ekhatipo at NOSPAMmidway.uchicago.edu
Wed Jan 10 12:39:51 EST 2001

Do you think my understanding of paralogs and orthologs is correct? I sort
of figured out a less professional determination, so I wonder whether my
view lacks some depth.
1) Paralogs are the genes with the same function belonging to the same
genome. E.g., E. coli has several genes for glutamine synthetase. So,
paralogs share horizontal functional homology.
2) Orthologs are genes with same function belonging to different genomes. As
a result of convergent evolution, orthologs not necessarily share
significant similarity (though they share similar functionally significant
domains). Thus, orthologues share vertical functional similarity.

"Simon Andrews" <simon.andrews at bbsrc.ac.uk> wrote in message
news:3A5B228D.2E78255 at bbsrc.ac.uk...
> 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
> alpha-rat.
> If all this isn't completely clear then take a look at the slide at;
> http://www.ncbi.nlm.nih.gov/Makalowski/Presentations/Purm_99/purm4.gif
> which may give you a better feel for the differences.
> Hope this helps
> Simon.

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