identical mutations

Robin Panza panzar at CLPGH.ORG
Thu May 8 10:15:13 CDT 1997

>Actually, I think something is missing here - if the mutant alleles are not
>identical but have similar effects, then they could NOT form homozygotes
>(by definition) if they came together in a single individual, which was
>what the original pop'n geneticist *explicitly* said was involved (because
>unless they *were* identical and *could* form homozygotes, each mutation
>would likely vanish from the population very quickly, being neutral in
>heterozygous form).

Not quite true, because of the redundancy of the genetic code (several RNA
triplets producing the same amino acid linkup).  Several different mutations at
the same site could effect the same amino acid substitution.  While these are
not identical mutations at the level of DNA base pairs, they are
*functionally* identical.  They can complement each other producing the same
biochemical phenotype in "heterozygous" form.  Either or both could spread in
the population.

However, I think the original post was about a slightly different phenomenon,
one that hasn't been addressed yet.  If overall mutation rate in the region of
a particular peptide's gene is high, it would produce many deleterious, some
essentially neutral, and some beneficial mutations.  The higher the mutation
rate, the more mutations occurring, and the greater the probability of the
same beneficial one more than once.  If a mutation is truly recessive (and
there are many degrees of and variations on dominance), it is functionally
neutral at low frequencies, because rare recessives make even rarer
homozygotes.  This can allow a recessive beneficial allele to remain in the
population long enough for a second, identical or complementary mutation to
occur.  This effectively increases the chances for a pseudohomozygote (don't
know the word for "heterozygous but complementary"; parazygote?), exposing the
allele(s) to selection.

Robin Panza                     panzar at
Section of Birds, Carnegie MNH
Pittsburgh  PA  15213  USA

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