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.

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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