> Somewhat confusingly for geneticists sticking to these terms, the apparent
> dominance and recessiveness can change according to how much of the chemical
> the organism is exposed to.
>

I also found the original to be confusing.  Gavin's reply was excellent, but
let me expand.

Say that a mite has an allele (form of a gene) for pesticide resistance.  At
a low dose of the pesticide, a single copy of the allele (from one parent)
would be enough to confer resistance.  In this case, we'd say that the
allele was "dominant."

However, at a higher dose, it might take two copies of the allele, so in
that case, we'd say that it was functionally "recessive."

Same allele of the same gene--recessive or dominant simply depending upon
the dose.

Where it gets complex is that many genes have more than one function, so
that when you change one allele, you may change other functions in the
mite--usually for the worse.  If pesticide pressure is continued, then there
will be pressure for the other genes to change, so as to again create a
machine whose parts all work together.

So once the mite has been exposed to a miticide for several generations in a
row, it may evolve to the extent that miticide resistance is *locked* into
the genome, and reversion would then recur slowly, if at all.

Randy Oliver

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