[gurps] BioTech-- Eugenics Question

[Susan Koziel]

--- "Chris J. Whitcomb" <rekres at gmail.com> wrote:

> David Scheidt wrote:
> > On Jan 26, 2008 7:10 PM, Susan Koziel
> > <kataryna_dragonweaver at yahoo.com> wrote:
> >>
> >> ---
> >>>  Does that number even
> >>> come close to modeling a similar circumstances
> in
> >>> humans?  How quickly would I have almost all
> 7-foot
> >>> humans if we only allowed seven-footers to
> reproduce
> >>> with each other, and nobody else?  Is it 1-2
> >>> generations, or 10, or more?  I think probably
> only
> >>> 1-2, to be honest.
> >>
> >> Height is a multigene thing that is largely
> influenced
> >> by environment. Breeding only 7ft isn't going to
> >
> > Dutch men are the current favorite example of
> this.  They're about 20
> > cm taller (average height is a bit more than
> 185cm) than they were
> > about 100 years ago.  Given that it is
> > rather unlikely that they've been engaging in
> selective breeding,
> > that's down entirely to factors like diet.
> 
> It all depends on how you define selectice breeding.
>  If the women of the 
> country on average prefer taller men, then for the
> most part the taller men 
> will breed...
> 
> I'm not saying it definitely is a factor, but that
> it can't be completely 
> ruled out.
> 
I'm not saying that height isn't heritable. 
It is. 
The issue is that selecting for it is not straight
forward, and it does not necessarily hold true that if
you take two tall people you will get taller
offspring. 
You might.
Or you might not. 

If you take two tall individuals you are just as
likely to get offspring that are the same height or
shorter then their parents. If you happen to stumble
on a set of parents that breed true and their
offspring are taller you will only get so tall before
the maximum effect of that combination of genes is
reached. After that you have to bring in another set
of genes - and that's a gamble... that set of genes
commonly cancels the original mutation (making your
offspring shorter) or in more rare cases (and this is
what you want) they will make your offspring taller...
but this taller may or may not be heritable. You need
to check the next two generations to know for sure....
then you will eventually max out the height of that
cross. Eventually you will actually max out height for
the species and you'd have to turn to genes from other
mammals.

This is what the plant people have encountered. Plants
have been breed and tinkered with over time more then
any mammal breeds. For example: With Canola and Soy
they have been trying for years to decrease the amount
of bad oil in the seeds... they maxed it out by
crosses fairly early on (because there isn't that much
natural variation in the crops). Now the only way to
change them is to either intentionally mutagenize
populations or bring genes in from other sources. 

There are indications with animal breeds that the same
effect occurs - that each species has a max level
within it's genome for certain things like size, IQ,
speed, etc. Much of these maximums have to do which
the sheer physical nature of our parts.
Too tall a human and you have both back and knee
problems - to push past these limits you need to bring
in a number of outside systems.

I digress.... (getting back on topic)

If you are talking natural selection and normal human
evolution - yes sex selection is a very strong push -
but this isn't something that happens in a small
number of generations.

What you are likely seeing in the Dutch community is
the increased prevalence of a mutation rather then the
effects of sex selection (sort of like Tay Sachs
disease being prevalent in certain ethic
communities)... it's not selection you are seeing -
but rather an increase or decrease in frequency of
certain genes within a semi-closed population. This
can lead to a trait changed by sex selection; but
since the Dutch can and do breed outside their
community ;) suggests that the current effect isn't
directly due to sex selection.

If you wanted to breed tall humans then starting with
a tall Dutch man from a family of tall Dutch men is an
ideal choice. Then crossing him with an average human
and comparing his gene profile to his wife's profile
then to his children's profiles (iding the children
being tall or short) is how you'd start to get a gene
profile for tallness. Once you have that you can start
looking at what happens when you combing different
tallness profiles.

Of course this sort of thing isn't the same as looking
at what happens in normal society (nor would I
consider it a proper use of genetics - since
variability in a genome helps a species survive in the
long term).

The term Selective Breeding, as I am using it, is when
someone forces a plant, or an animal to breed with
chosen plant or animal. So that the experimenter can
track the parentage.

The term Natural selection, means what happens in
nature, without the interference of an experimenter.
Sex selection is a factor in natural selection... not
in selective breeding. (As I am using the terms).

I realize these terms seem to be splitting hairs - but
the different terms in genetics are important because
they tell you which factors you are looking at and
which ones you can discount when you look at a system.
-Sue