Re: [MLUG - DISCUSSION] cloning

["Jonathan King" <EMAIL:PROTECTED>]

On 10/5/06, Mike Miller <EMAIL:PROTECTED> wrote:
> On Thu, 5 Oct 2006, Vern Green wrote:
> >
> > Additionally one would have to wonder about Color blindness. I am color
> > blind and the way color blindness works is it skips a generation, but it
> > is hereditary in nature.
>
> X-linked recessive.  That means that you cannot pass it to a son, but you
> can pass the gene to a daughter.  A daughter will not be color blind
> though, usually, because she will have a good copy of the gene from her
> mother.  But the daughter is a carrier, so 1/2 of her sons would be color
> blind.

The explanation for most cases of colorblindness is actually very
interesting. Specifically, the reason why it happens at all is related
to the fact that people are trichromats (have three different
photopigment genes) . Two of these genes are on the X chromosome, and
they are literally right next to each other, a so called "tandem
repeat" which was apparently caused millions of years ago by a
mis-copying event. With time, mutations were introduced into both
copies of the gene (which continued to function), but some mutations
caused the photopigment to be more sensitive to longer wavelengths,
and others to shorter wavelengths. It turns out that it's pretty clear
there is some advantage in having three rather than two photopigments
(it makes ripe fruit easier to see against green leaves, for one), so
people with three distinct photopigments ended up having more
surviving offspring.

So how does colorblindness happen? I believe the current explanation
is that one of the genes can be pretty easily lost by a phenomenon
called "uneven crossing over". In mega-brief, when the stem cells that
become germline cells divide, they swap genetic material from one
chromosome to the other pretty easily because the sequences are
incredibly similar and the distances involved are small; a fancy name
for this is homologous recombination. But now suppose that one
chromosome in the area of the red and green pigment genes has a bend
in it during crossing over. Because the two genes are tandem repeats,
the sequences are very similar even if you "skip" one of the genes, so
when you swap material, one chromosome can be left without one of the
two genes.  And the result is what you see in Vern, or rather in what
Vern *doesn't* see.

> > I guess what I am asking is if the trait manifested itself with me,
> > would it by nature HAVE to manifest in my clone? This was not something
> > that was externally influenced, but does a trait HAVE to manifest for it
> > to be a full clone?
>
> If the kind of X-linked recessive color blindness that you have has 100%
> penetrance, then your clone will also be colorblind.

See the above. Since guys have only one X chromosome, they will be
colorblind if their mom had one chromosome with the deleted gene. As
it turns out, having only one pair of functioning photopigment genes
tends to cause guys to be less sharp generally at discriminating
colors, because they are more likely to be affected by other mutations
that occur in the red or green locus that can lower sensitivity.

Well...gotta get back to work.

jking

_______________________________________________
discussion mailing list
EMAIL:PROTECTED
http://mlug.missouri.edu/mailman/listinfo/discussion