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ortholog and paralog

Paul Linehan linehan at indigo.ie
Sun Feb 23 09:56:17 EST 1997

pryor at mail.teclink.net wrote:

> duret at acnuc.univ-lyon1.fr (Laurent Duret) wrote:

> >   "Where the homology is the result of gene duplication so that
> >   both copies have descended side by side during the history of 
> >   an organism, (for example, alpha and beta hemoglobin) the genes
> >   should be called paralogous (para = in parallel). Where the 
> >   homology is the result of speciation so that the history of the
> >   gene reflects the history of the species (for example alpha 
> >   hemoglobin in man and mouse) the genes should be called orthologous
> >   (ortho = exact)."

> OK, fine. But how do you know the beta globins are homologs? Could you not 
> have duplication of beta globins in an ancestral species, with one lineage 
> maintaining functionality in one and the other the other..with genomic 
> rearrangements, etc. how could you tell..or due you assume that there was 
> no duplication of beta globins.. an assumption which appears to be 
> absolutely incorrect.

A way of testing this might be blotting under non-stringent conditions
to see if any piece of DNA in the organism under consideration matches
the beta globin sequence sufficiently to be considered related.

> How could you ever possibly know that "a gene reflects the history of a 
> species" without making a prior assumption that the gene you want to 
> "reflect the history of the species" does reflect the history of the 
> species?

Ultimately there is a certain amount of "relativity" in all of this. 

Genes which are paralogous for one set of species being compared may
be orthologous for another. The way to determine which might be the
case is to use other bits of evidence, the fossil record and
morphological studies to put your genetic study in the correct context
and of course the coverse can also be done.


> Robert Hamilton

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