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homologue vs ortholog

Joseph A White whitejo at PILOT.MSU.EDU
Wed Sep 4 13:46:03 EST 1996


After reading the responses to Lee Neisel's question, I think a little bit
of proper terminology needs to be injected here.  Most of the terms that have
been used are in fact jargon.  Using them is fine, but as is evident,
confusion results.

Geeta Bharathan is correct when she points out that homology, and the two
forms of homology, orthologous and paralogous, are evolutionary concepts
used to describe ancestral relationships of genes.  I don't know whether the
terms "orthologue" and "paralogue" are actually nouns or not, but there is no
mention of them in any text that I have consulted.

> The terms orthologue and paralogue were introduced by Fitch in the 1970's, I
> believe. He used the terms to describe different kinds of duplicated
> genes--duplications within the genome of a species lead to paralogous genes;
> duplications due to speciation lead to orthologous genes."

These relationships are correct, though: if gene1 and gene2 are homologous,

            _______Gene1                   ______Gene1
Organism A <_______            Organism B <______
                   Gene2                         Gene2

Gene1 in organism A is paralogous to gene2 in organism A and organism B, and
is orhtologous to gene1 in organism B.

Todd Richmond gives essentially the same analysis, except for introducing the
term "metalog", which has no root word (i.e. orthologue from orthologous) and
is merely jargon.  The relationship to which this term refers is a paralogous
relationship, i.e. gene1 in organism A to gene2 in organism B.

Geeta Bharathan also is correct in pointing out that we need more information
than similarity to infer homology:

> I believe that molecular biologists often use functional/other experimental
> criteria to determine orthology.

Homology requires that genes or structures (fosil structures) be similar by
descent.  Similarity alone cannot provide precise information on ancestral
relationships, even though we use similarity for that purpose all the time.
We infer homology from similarity.

So here is a question for you all: without having "other experimental
criteria", how does one deduce whether two genes from a small
gene family in two different organisms are orhtologous or paralogous?  By
similarity alone?

To return to the original question, the terms that I have found in texts are
these: homology, homologous, orthologous, and paralogous.  I can only assume
that the others, although useful at times, are merely jargon.

Joe White

(whitejo at pilot.msu.edu)
301 Biochemistry
Michigan State University
East Lansing, MI  48824

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