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?

ron o

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

laurens

The whole story is somewhat complicated. First of all, the Y-chromosome
is a special one in the whole range. Men have one Y and one X
chromosome, women two X-chromosomes. This difference determines our
gender. The Y chromosome is a shorter version of the X chromosome. When
speaking about dominant and recessive, it is confusing to speak about X
and Y chromosomes because certain genetic information is not found on
the Y chromosome, only on the X chromosome.

Chromosome repair (You mean DNA repair I presume?) is conducted with
the other strand in the same chromosome. So per chromosome you have two
complementary strands. The other chromosome contains the same genes,
possibly in a somewhat different form (variant). This is the level
where dominant, recessive and intermediate gene expression takes place.
That is between two chromosomes of the same chromosome pair.

So when an error occurs, the DNA will (most of the time) be repaired
succesfully with the other strand from the same chromosome. The second
chromosome will not have any function in this process.

To return to your question:
When an error occurs early in development, and it can not be repaired,
a gene can shut down. The gene variant on the other chromosome can take
over giving rise to a cell line with somewhat different properties. It
can happen, though it is very rare. And yes, it can give some weird
results. For example: I know someone with a radially extending dark
spot on his iris. One cell has had a mutation and it has spread in that
particular cell line.

I hope my answer is understandable.

Laurens

melody_ann_ryan@yahoo.com schreef:

perplexed in peoria

melody_ann,
Your understanding is correct. For another (common) example of a genetic
chimera try Googling on the words:
calico cat barr body