homolog vs paralog?

Chuck Staben staben at pop.uky.edu
Sun Feb 23 22:22:48 EST 1997

The distinction between homolog and paralog is best illustrated, for me,
by an example.  Compare human and mouse globins.  All are clearly
related in sequence, function, and evolution.  Mouse alpha and human
alpha are homologs because they presumably have a common evolutionary
origin in the alpha-globin of the rodent-human common ancestor.  Mouse
alpha and human beta globin are paralogs because their most recent
common ancestor is presumably the ancestral globin gene,
pre-duplication.  This duplication predates the mouse-human divergence.
If you were looking at ONLY the mouse alpha and human globin, you might
infer that these were homologs-so determining homology and paralogy
seems very dependent upon the data available and even upon the
evolutionary history.  (If the mouse had lost its beta globin gene and
human had lost alpha-this would not change the homology-paralogy, but
our inference would probably be very different.)

It seems to me that within a species, genes that have similar sequences
due to a common evolutionary origin might be either paralogs or homologs
of each other.  If they are homologs, they must have diverged from one
another after the divergence of that species from its closest relative.
If they are paralogs, the evolutionary divergence of the paralogs
predates the divergence of this species from the other species under
consideration-I think.
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