Wow! I can feel the rust slowly breaking loose from my brain. Its been a
LONG time since I had to think about this stuff and thanks for taking the
time to answer me. I am guilty of confusion and oversimplification, stemming
from sloppy thinking.

If we take your's and Steve's hypothetcial and agree that your math is
correct we see that the probability that an offspring would have *no*
genetic material from a grandparent to be 0.0009765625

My question is: Isn't that probability kind of small? Or maybe my question
is: Is that a small probability? Is that what you meant by Helmut
overstating his point?

K Barrett

"Ted Byers" wrote in message
om...
Steve

It is possible to make the explanation simpler by using a little math.
It is really a simple problem in probability theory. In your
example, each individual gets ten chromosomes from each parent, and so
in its progeny, there is a probability of 0.5 that a given chromosome
came from a given parent. Therefore, with your numbers, you can
compute exactly the probability that a given individual has virtually
no genes from a given grandparent: 0.5 raised to the tenth power, or
0.0009765625. Given that a given mating of orchids may produce
hundreds of thousands on seeds, it is probable that there are some
progeny that have no genes from a given grandparent. You could go
even further and compute the probability that there are no progeny
that have no genes from one of the grandparents, or the probability
that there are progeny lacking any contribution from one maternal and
one paternal grandparent (0.00000095367431640625: I'll leave the
details of getting this answer to the reader ;-).

Of course, if funds are not limiting, one can determine the relative
contributions from the more distant ancestors using DNA
fingerprinting, but that is a whole other story.

K...., I know I didn't tell you one thing about genetics that you didn't
already know. Somewhere you took Helmut Rohrl to mean something more
than he did, I think.


If he was quoted accurately, then he overstated his point.

Last of all, I know genetics gets more complicated with genes having
ways to migrate to different chromosomes so that one chromosomes will
actually contain genes from more than one parent. There, also are the
plants that end up as 4n or 6n; multiple copies of each chromosome and
more potential to carry genes from more ancestors. That doesn't matter
much. The above is still valid. I just hope I didn't confuse everyone.
I'm no teacher!

Actually, it does matter plenty. Work the numbers and you'll see that
increasing polyploidy significantly reduces the probability that a
given individual lacks a genetic contribution from one of its
grandparents, or more distant ancestors

The effect of crossing over (a process where a pair of chromosomes
exchange genes) is much less significant, unless it is much more comon
that I have understood it to be.

K.

I *think* I'm having trouble because I'm used to animal genetics where
a
limited number of ovae are fertilized and offspring without genes from
a
grandparent might not survive past the blastula stage (*G*). However
in

This is a possibility regardless of the genetic contributions of the
ancestors and the number of offspring.

orchids, hundreds of thousands of eggs can potentially be fertilized.
Therefore the mathematical realm of possibilities (bell curve or gene
segregation ) can be demonstrated.

Well, I am not sure I'd put it this way. The probability of the
event, in a given indivudal, is not affected by the number of progeny,
only the probability of observing it in a given population at a given
time: these are two different probabilities. I am not sure that the
bell curve applies. I would have thought a poisson or a binomial
distribution more likely; but the I haven't had enough interest in the
question to actually work out the correct probability distribution for
independant assortment.

I still think, however, that in practice the lack of genetic material
would
lead to the decreased fertility seen in complex hybrids (ie its not
unususal
for some plants to be poor parental stock or for some crosses to yeild
a
minute number of seed or flasks). And that therefore there would
indeed be
genetic material from all parental stock in the exisiting offspring ,
no
matter how far removed.....

I doubt that a lack of genetic material is to blame here. Rather, I
suspect if cultivars with different numbers of chromosomes are
involved in the cross, the probability of the cross having an odd
number of chromosomes increases, and then the cross becomes less
fertile being the process of segregating the chromosomes being two
gametes no longer works well. This is why sexual reproduction tends
to decrease in importance and asexual reproduction tends to increase
in importance as the degree of polyploidy increases: but highly
polyploid plants tend to be incredibly robust and difficult to kill.

But none of this affects the probability that in surviving progeny of
a cross that is known to have low fertility will have a contribution
from a given ancestor: only the probability that it will be observed
in a given population at a given time.

Am I wrong? Or am I right in why I'm wrong?

I expect to be wrong because Rohrl is much more brilliant than I am.
And
when I was in school genetics was taught simplistically to us
undergrads and
DNA theory was in its infancy (the one gene one trait idea is now out
the
window, for example)

Now if you were my student, I would yell at you for such a remark.
DON'T SELL YOURSELF SHORT! And don't over estimate the capability of
alleged "experts"! I can tell you, as a theoretical ecologist, every
population genetics model I have seen leaves just about everything to
be desired, and are really useful ONLY for the purpose of explaining
some basic processes to undergraduate students. Real life is much
more interesting and complex than they suggest, but I have not met a
geneticist who is really up to the challenge of developing a
population model that is of practical utility. There are few
ecologists or environmental scientists who are up to that either, but
I am working hard on changing that. :-) I met one recently who, even
though working in an engineering aspect of environmental science, was
so mathematically challenged that she couldn't see the obvious way to
compute the spatial distribution of contaminants from a linearly
distributed contamination source (think fumes from cars an roads)!
Neither could her graduate students. And yet SHE is regarded as an
expert! I have two earned doctorates, and so I know what it takes to
become an "expert". Trust me when I tell you that barring the most
severe developmental handicaps, anyone can do it with just a little
effort. Sorry, about this rant, but I find self deprication most
irritating (except, of course, when it is me reviling myself: but
then, it is OK since I REALLY am a moron :-).

Cheers,

Ted