>recessive "defective" genes where simply bred out of the "bloodline"
>
>You don't find two separate corn growers interbreeding their corn and
>suddenly finding a crop with the same problems crops had thousands of
>years ago, because there is simply no recessive genes for that
>particular problem still in existence.
 
Not exactly.  Current farmed monocultures turn out to be MORE vulnerable
to unexpected challenges like novel illnesses, not less vulnerable to
catastrophic loss.  That is due to genetic narrowness and due to being
extensive populations with genetic narrowness (kind of like the way that
having too many households with one operating system or one browser which
is vulnerable to computer viruses and computer worms allows the "disease"
to spread like wildfire), though at times, yes, the crops have better
resistance to certain specific illnesses, so that part of your point is
a very valid one and very worth consideration but just shouldn't be
generalized, that's all.
 
I was dragged kicking and screaming into several type of knowledge (like
using computers) so I have to admire folks who come here to share and
learn.  I am rusty and have found myself having to refresh things i
recall and learn things I have forgotten so very often.  There is a
precarious stack of texts next to my chair and I swear one day I will go
into Google and the screen will read, "What!  You AGAIN?".
 
The importance of genetic variation is why there has been acceleration of
multiple nation's efforts to bank seeds and pollens of heritage crops and
of wild relatives when those can be found.  There was incredible joy in
the last few years when a wild corn relative was found in a small
population and in one area long farmed by an isolated population several
heritage potatoes were found.
 
(BTW, my last severe reaction was to something which may have been a
genetically modified vegetable.  I really wish they would mark those
things.  Now we'll have to try me on small slices of similar crops,
increasing amounts gradually, which may have less of a chance of being
modified and see if I react and to what extent.  It would be so silly
to die just because someone didn't want to put up identifying signs on
such veggies so prevented the labeling.  Anyway, genetic modification
is one way that horticultural geneticists are introducing some types of
resistance.)
 
Anyway, when we selectively breed we often reduce genetic variation and
that takes away protection from crashes due to unexpected illness: true
in ferrets as well as in corn.  (Though a breeder who is good at keeping
health and longevity records and tries to do so for ALL line members (or
with at least as many of the new homes as will share) can most definitely
breed for health and longevity and be largely successful.  It would be
wonderful if the important things like health and longevity were
considered far more important than looks by more and more breeders and if
breeders learned more.  And here I want to give thanks to the marvelous
folks in breeders' groups who go to the trouble to try to educate
breeders to keep good health and longevity records, and to carefully
avoid breeding some ferrets.  You have earned everyone's thanks each
time you warn against dangerous breeding practices!
 
(BTW, if anyone noticed the comment about rice having far more genes than
humans and was wondering: it's hypothetical but here is one possible
reason: remember that rice can not pull up roots and move somewhere else
when something major (an illness, a flood, etc) happens, so having broad
genetic variation in individuals as well as in the population and having
many places to store variation for different circumstances gives a
population more of a chance to weather a challenge instead of dying out.)
 
Oh, one other point.  Sometimes a vulnerability isn't removed but a
resistance is increased and that off-sets the vulnerability.  Many things
involve more than one genetic location feeding into the final product.
Hypothetically, you can get a gradual change in a population by changing
how the alleles interact by changing their proportions in a population,
or you can get a rapid and larger change by doing things like altering
the proportion of an allele which affects one of the fetal groups of
cells which develop into multiple cell types, as with neural crest
medical disorders.  (Other routes exist.)
 
Darrin, your math is dead-on for recessive genes.  You can have them
present in a population but not show up because they are hidden enough
behind more dominant genes, and in that case the percentages of the
alleles will become stable at a certain low level -- without effective
enough selective breeding or a major event like a pandemic which changes
proportions, so those recessive can not wind up lost completely.  Many
neural crest genetic disorders are from dominant genes with variable
expression.
[Posted in FML issue 4670]