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homologue vs ortholog

Todd Richmond trichmon at students.wisc.edu
Tue Sep 3 19:26:40 EST 1996

In article <322B1B95.330C at aesop.rutgers.edu>, meisel at AESOP.RUTGERS.EDU wrote:

>Recently when reading a variety of papers I have seen ortholog and
>homologue used in similar context.  I'm pretty sure there is a
>difference between ortholog and homologue but am unsure of what that
>difference is.  Unfortunately, ortholog is not in the Webster's
>dictionary or in a genetic dictionary we have available.  Could someone
>please explain the difference between ortholog and homologue, as well as
>the criteria necessary to define a sequence as an ortholog or a
Since no one else has leaped in, I'll bite:

Start with Gene A.
Duplicate Gene A to get Gene A'
Hit speciation point.  Now you have two species, both with a Gene A and a Gene 
A'.  Call the four genes 1A, 1A', 2A, and 2A'
There are three different types of homologous comparison possible when 
comparing these gene combinations.
1A and 2A are orthologs, same gene in different species
1A and 1A' are paralogs, duplicated gene in same species
1A and 2A' are metalogs, one gene compared to the duplicated gene in a 
different species.

In the generic sense, orthologs, paralogs and metalogs are all "homologs" but 
the one most commonly used is the orthologous relationship.

For comparing genes you want to use the orthologous genes of 1A and 2A.  You  
really shouldn't use 1A and 2A' to do comparisons.  Looking at the paralogs is 
useful if you want to determine the approximate time of duplication of the 

Hope this helps,


Todd Richmond                  trichmon at students.wisc.edu
B129 Birge Hall                UW-Madison Dept of Botany      

"It's better to ask forgiveness than it is to get permission."
The late Rear Admiral Grace Hopper

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