[Dragaera] Breaking the Cycle - Genetic Purity

[Jerry Friedman]

--- Maximilian Wilson <wilson.max at gmail.com> wrote:

> On 8/28/06, Gaertk at aol.com <Gaertk at aol.com> wrote:
> > "Maximilian Wilson" <wilson.max at gmail.com> writes:
> >
> > >[Cochran's idea is that if you take evolution seriously, not many
> > >diseases can be genetic in origin because they'd quickly get bred out
> > >of the gene pool; therefore we should be looking for environmental
> > >causes like pathogens, which are transmitted independently of host
> DNA
> > >and therefore not subject to evolutionary pressure in the same way.]
> >
> > That only works if the disease is the sort that would prevent you
> > from reproducing.  My parents lived 60+ years and had three kids
> > before finding out Dad was born with a defective heart valve and Mom
> > had a cyst growing in her brain.
> 
> 
> Well, it only has to have an effect on reproductive fitness. Even if a
> defective heart valve is fatal in early childhood only 1% of the time,
> or a cyst in the brain makes you only slightly less likely to provide
> well for your offspring, over 1000 generations it will get bred out
> fairly quickly, and therefore cannot be purely genetic in origin. Or
> so one would expect. 

[snip interesting stuff about toxoplasmosis]

More precisely, you reach an equilibrium where the rate of
disappearance of the gene by selection balances the rate of
new mutations.  I had the impression that this is why cystic
fibrosis is relatively common--the gene is big and susceptible
to mutation.  (Wikipedia says that the disease is "up to 52,000
years old", which is more than 1000 generations, that the
theory that heterozygosity improves resistance to cholera was
not confirmed by later studies, and that the reason for the high
prevalence among whites is unknown, so my impression might be out
of date.)

Of course, this makes one Cochran's "not many diseases" that are
purely genetic in origin.

Then there's the definition of "disease".  I "suffer" from
anomalous trichromatism, which apparently has only a tiny effect
on fitness.

Jerry Friedman has trouble with greens and browns.  Also purples.

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