Daniel & Monica Glaeske daniel.monica.glaeske at
Tue Sep 22 20:26:47 EDT 1998

Kenelm Philip wrote:
>         John Grehan said:
> > My understanding (which I admit may be inadequate) is that the gene
> > from one parent will convert the gene from the other so the offspring will
> > not be heterozygotes.
> That's a far more unorthodox idea than directed mutation! My first question
> would be: Why does the mutated gene convert the un-mutated gene rather than
> vice-versa (assuming that this process occurs at all)? My second question
> would be to inquire what this idea does to the standard explanation of the
> advantages in Africa of being a heterozygote for sickle-cell.
>         Modern genetics has come a long way from Mendel--and there are some
> very complex things going on. I had not heard about 'mutation by
> association'
> transforming genes in this manner, however. Any references for this idea?
> As far as I know, the concept of heterozygosity is still valid...
>                                                         Ken Philip
> fnkwp at

Actually, there is some validity to genes "transforming" other genes
(actually alleles) in the realm of bacterial genetics.  Most "mutant"
genes (whether they are beneficial or not) almost always are less
ergonomically efficient than "wild-type" genes.  If you have, for
example, a gene coding for antibiotic resistance (say erythromycin
resistance, which codes for an altered ribosomal subunit not susceptible
to erythromycin, but has less affinity for either tRNA or mRNA, I can't
remember which) on the bacterial chromosome, and the "wild-type" gene on
a plasmid, the "wild-type" gene will "cross-over" and reestablish the
wild-type gene on the chromosome.  It's probably not as simple as
Mendelian crossover, may have something to do with rRNA from the
wild-type gene hybridizing with the the "mutant" allele during DNA
synthesis.  However, this is more likely to occur in the absence of
selection pressure for the mutant gene, so it does not remove the
influence of natural selection (however unnatural it is under laboratory
conditions).  I can't recall the specific references for it, but we came
across it when we were purifying a protein from E. coli, linking it to
an antibiotic resistance gene.  Our paper may or may not have discussed
this phenomenon, I can't remember.  If you're interested, I'll see if I
can pull up any other references.  


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