Text 29, 84 rader
Skriven 2004-09-12 06:14:00 av R Norman (1:278/230)
Ärende: Re: Genetic Drift and Mut
=================================


On Sat, 11 Sep 2004 20:34:10 +0000 (UTC), Brett Aubrey
<brett.aubrey@shaw.ca> wrote:

>Merriam-Webster Online has the following definitions:
>
>genetic drift
>: random changes in gene frequency especially in small populations when
>leading to preservation or extinction of particular genes
>
>mutation
>: a relatively permanent change in hereditary material involving either a
>physical change in chromosome relations or a biochemical change in the
>codons that make up genes; also : the process of producing a mutation b : an
>individual strain or trait resulting from mutation
>
>My Questions:
>
>1. Are the random changes involved in genetic drift not also relatively
>permanent?
>
>2. How can a change involved in genetic drift lead to preservation?
>
>3. How does genetic drift manifest itself in the real world (as I think I
>understand how mutation can manifest itself.)
>
>4. Can anyone improve (or elaborate for a layman) on the Webster definitions
>to allow a better understanding of the differences between these terms, or
>are the definitions as good as it gets.  (Perhaps to understand the
>differences I need to know in detail how gene frequency, codons, alleles,
>chromosomes, genes and more are related to each other and I don't have that
>training,)
>


There are several things at work.  You may be missing something and
the definitions are definitely somewhat flawed.

Evolution is generally defined as something like "a change in the
genetic composition of the population from generation to generation."
There are several aspects to the "genetic composition" that can
change.  Ordinarily there is some variability in the specific DNA
sequence from individual to individual.  That is, each gene can exist
in several different forms or alleles.  The "genetic composition"
really refers to the relative abundance or frequency of each allele
for each gene in the population.

In a small population, it is quite possible (even very likely) that
the abundance of a particular allele might change just be chance over
generations.  For example, those individuals with big noses just
happened to have more babies with big noses than expected.  It is sort
of like flipping a coin twice. You expect one head, one tail, but
sometimes you end up with two heads. That is the definition of genetic
drift: a change in the genetic composition of the population (in the
allele frequencies).  If the change is drastic enough, it can in fact
lead to the elimination or the fixation of an allele: so few people
with small noses happen to have small-nosed babies that the small-nose
allele simply disappears and the big-nose allele is then "fixed" in
the population.  The flaw in the definition is that drift is any
random change in gene frequency whether or not it leads to
"preservation or extinction" of a gene.

As a completely separate matter, an "error" might occur in cell
division that causes a change in the sequence of DNA or in the
structure and arrangement of the genes along a chromosome (or even in
a rearrangement of the chromosomes, themselves).  Once such a change
occurred, a mutation, it would then be faithfully copied (in the
absence of another mutation) and so is a "permanent" change.  The
second flaw in the definition is that inappropriate emphasis on the
permanence of the mutation.  The important thing is that the change is
the introduction of a completely new variation into the population,
not the change in frequency of existing alleles.

For better definitions, consult Wikipedia, usually a good reference
for many things. Googling "genetic drift mutation" will give you many
academic sites with instructional guides to these topics, too.
---
ū RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info@bbsworld.com

---
 * RIMEGate(tm)V10.2á˙* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 9/12/04 6:14:23 AM
 * Origin: MoonDog BBS, Brooklyn,NY, 718 692-2498, 1:278/230 (1:278/230)