homologue vs orthologue?

[Mark Fry]

Can somebody please tell me the difference between homologous genes,
orthologous genes and paralogous genes.

Thanks

Mark Fry
rm 4330 BMS
Faculty of Medicine
Memorial University of NFLD
NFLD, Canada
A1B 3V6
wm...@plato.ucs.mun.ca

An idealist is one who, on noticing that a rose smells better
than a cabbage, concludes that it will also make better soup.

H.L. Mencken (1880-1956)

[Simon Andrews]

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.

[Emir]

Simon,
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.
Emir

"Simon Andrews" <simon....@bbsrc.ac.uk> wrote in message
news:3A5B228D...@bbsrc.ac.uk...

[gavin]

Emir,

There is an article in Trends in Genetics (1997) 13, 432-433 by Anouheif et al. which gives
more details on what you've described. To briefly paraphrase, orthologous genes are
useful to find out evolutionary history between species while paralogous genes can tell
you more about a gene duplication event.

About speciation vs duplication, here is another good illustration
http://www.stdgen.lanl.gov/stdgen/bacteria/analysis/MYCO/orthologdef.html
Graur & Li (in their book, Fundamentals of Molecular Evolution [2000], p271) also give a
succinct definition of orthologous and paralogous genes.

Gavin

[Errol]

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.