Re: chromosome repair leading to expression of recessive genes?
- From: "Ron O" <rokimoto@xxxxxxx>
- Date: Wed, 7 Sep 2005 00:38:45 -0400 (EDT)
melody_ann_ryan@xxxxxxxxx wrote:
> I was reading this article talking about the future of the Y chromosome
> and this paragraph interested me:
>
> http://www.npr.org/templates/story/story.php?storyId=4225769
>
> "All the other chromosomes come in two copies. Every time a cell
> divides, mistakes in genes can creep in. In paired chromosomes, that
> means that if there is a mistake on one chromosome, a cell can always
> get the correct gene sequence from the other chromosome."
>
> Sorry if I am making a big misunderstanding, but it sounds to me that
> when a gene on a chromosome is damaged, the chromosome will just repair
> it with the gene on the other half of the chromosome. If there was a
> gene pair where the dominant gene was damaged, would the cell (and all
> of its offspring) then express the recessive gene? That could lead to
> some wierd results if it happened early in development. Am I off base?
There are different reasons why one allele is dominant over another
allele. You have things like haplo insufficiency where if you destroy
the function of one copy of the gene and you have only one functional
gene it can't produce enough gene product to create the usual phenotype
so you can observe a change in phenotype due to the knockout. You can
have parts poisoning where the defective gene product is part of a
complex that is disrupted by the defective gene product so you observe
a different phenotype. The usual reason for dominance is that the
fully functional gene copy can perform its function well enough so that
you can't tell that the individual carries a defective copy.
You can get gene repair from the good copy by gene conversion, but this
is pretty rare.
It sounds like you are asking a question about heterozygous cells that
have one recessive allele and a dominant allele and what happens when
the dominant allele is knocked out. Take the last case where the
dominant allele is just the functional allele. If the functional
allele is knocked out in a cell you will express the recessive
phenotype in those cells because you now have two defective alleles.
The cancer genes like those associated with breast cancer incidence are
such examples. In a normal cell with two functional copies a single
mutation that knocks out one copy leaves the other still working, but
if you inherit a defective copy all your cells only have one functional
copy of the gene. A single mutation can now remove the function from
the cell and make it more likely to form tumors. So in these cases you
see the expression of what you are talking about by higher cancer rates
in the people that inherit the genetic defect. Most people with two
functional copies have the risk squared compared to the the people with
only one function copy.
Ron Okimoto
.
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