Text 1066, 135 rader
Skriven 2004-12-17 21:43:00 av Syvanen@ucdavis.Edu (1:278/230)
Ärende: Re: Exactly what are "rec
=================================



Wirt Atmar wrote:
> Mike writes:
>
> > >1) to obtain a clear explanation of the meaning of "recessive
genes".
> >
> >No such thing.  You want to know the meaniing of "recessive
mutation".
> >It has a very simple definition and is posed in classical genetics.
> >The phenotype of the heterozygote (mutant versus wild-type) is
> >wild-type.  If the phenoytpe were that of the homozygous mutant,
then
> >the mutation would be dominant.  Most of the answers in this thread
> >confuse underlying molecular mechanisms for this result with the
result
> >itself.
>
> While your answer is of course correct, it is also merely a
phenomenological
> description, absent of mechanism. As a result, it doesn't explain why
some
> alleles are recessive and others are dominant, any more than
observing that the
> sun travels across the sky during the day explains celestial
mechanics, thus it
> is fundamentally unsatisfying.

Of course I am correct.  Definitions of 'recessive' and 'dominant' are
defined by classical genetics.  As it turns out, the underlying
mechanisms for these phenomenon are extremely varied.  I have taught
genetics for nearly 30 years and have found that students frequently
confuse the basic observations with the underlying mechanisms.  As we
have witnessed in this thread.  The main point I was trying to make is
that the term "recessive gene" is quite meaningless.

Mike Syvanen

>
> We've known for a century that if the phenotype were that of the
homozygous
> mutant, the mutated allele would be declared "dominant," but that
knowledge
> still leaves us shy of an answer to the "why is that so?" question.
>
> In that regard, km34 writes:
>
> >Although your description is apparently sound, there is no necessity
> >for recessive mutations to be limited to enzymatic genes, nor are
> >mutations in enzymes limited from being dominant.  Similarly,
classical
> >"structural" genes may be mutated to act as dominant or recessive.
> >There is little correlation between dominance/recessivity and
> >enzymatic/structural.  In fact, as we get to know more and more
about
> >how genes act, the dichotomy of enzymatic and structural means less
and
> >less.  There are genes that code for proteins that are both, and
there
> >are jobs that some gene products do that are really neither
enzymatic
> >or structural.
>
> Let me stand by my original response. Fortunately, I'm not the only
one to say
> it. Victor McKusick has compiled an encyclopedia of basically every
human
> congenital defect known, and he has said this:
>
> "It now appears that these two categories [recessive and dominant]
correspond
> pretty closely to the two fundamental categories of proteins:
enzymatic and
> structural."
>
>      -- http://www.hhmi.org/genetictrail/e130.html
>
> But it also makes sense mechanically. Biochemical reactions
(substrate to
> product) occur spontaneously. However in the presence of a simple
catalyst
> (heavy metals, e.g.), they can be sped up thousands of times. But in
the
> presence of the complex electric field of a highly evolved protein,
their
> reaction rates often occur billions of times more rapidly than the
background
> rate.
>
> The active subunit of an enzyme is often quite small when compared to
the mass
> of the protein itself and its effective conformation sits in a very
stenotypic
> adaptive well. If anything occurs to the disturb the structure of the
enzyme's
> active site, catalytic function simply disappears, thus the most
common form of
> mutation-driven failure in enzymes is for them to simply cease
working.
>
> Because a "backup" copy of the functional enzyme may exist on the
alternate
> chromosome, such defects inherently tend to be "recessive" (that is,
the
> defective copy doesn't poison the reaction; it simply now has no
effect on it.
> If it did, the defect would be "dominant").
>
> But that's not nearly as true of structural proteins. Both copies, if
the
> defect is not too great, tend to be translated into protein
structures, but the
> combination of the two products, one standard and the other
defective, often
> prove to be "toxic" to one another and thus the defect is labeled
"dominant."
>
> There are of course exceptions to all of these statements. We are
after all
> talking about defects, and there are an infinity of more ways to
screw a
> process up than to make it functional, but the general statements
seem to hold
> quite well.
> 
> Wirt Atmar
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