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homologue vs orthologue?

Errol E.S.Kwan at Massey.ac.nz
Wed Jan 10 16:45:21 EST 2001

Paralogues do not have to have the same function, but have to show
some level of being homologues.  For example I think it's a human
globin gene and a human immunoglobin gene that have a high level of
similarity but different functions.  Because of the high level of
sequence  similarity then homogeneity can be inferred, even though
they have different fuctions.

On Wed, 10 Jan 2001 11:39:51 -0600, "Emir"
<ekhatipo at NOSPAMmidway.uchicago.edu> wrote:

>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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