X chromsome inactivation

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Bio-Hazard
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X chromsome inactivation

Post by Bio-Hazard » Thu Apr 13, 2006 10:43 pm

I've been trying to understand X inactivation; I haven't been able to figure it out.

From what I think I understand, X inactivation has to do with repressing an allele that gives out a certain phenotype.

What I don't understand is how calico cats have different colors of fur.
I was reading part of wikipedia and I'm assuming this explains why both genes are expressed in calico cats.

Expressed genes on the inactive X chromosome
- http://en.wikipedia.org/wiki/X-inactivation

During cell differentiation, some cells activate the allele that was packed away?

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Post by kiekyon » Fri Apr 14, 2006 5:41 am

The calico appearance, or phenotype, results from X inactivation. The genes controlling feline color coat reside on the X chromosome. As you are probably well aware, females have two X chromosomes, (XX), while males have only one (XY). During X-inactivation parts of each X are turned off to leave the equivalent of one functioning X. The genes for black, white and orange coat color reside on the X. Depending on which genes are turned off a specific region will be back, white or orange.

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Post by 2810712 » Fri Apr 14, 2006 6:46 am

so should we consider us & felinians diploid in context of X chromosome exprssions..?


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Post by Bio-Hazard » Fri Apr 14, 2006 12:25 pm

so should we consider us & felinians diploid in context of X chromosome exprssions..?


I'm talking about the cat that occurs after fertilzation. (haploid)

kiekyon wrote:During X-inactivation parts of each X are turned off to
leave the equivalent of one functioning X. The genes for black, white and orange coat color reside on the X. Depending on which genes are turned off a specific region will be black, white or orange.


1. When would X-inactivation of the fur color allele occur in calico cats?

While reading a text, the understaning of mine was that this inactivation occurs during cell differentiation?

2. Why are some alleles turned off while others are not?

Is the inactivation of alleles random to each cell?

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Post by LilKim » Fri Apr 14, 2006 11:18 pm

X-inactivation occurs as a dosage compensation mechanism in females. (Because a woman has 2 "doses" of the genes on the X chromosome.. whereas a male only has 1 X or "1 dose of genes).

In order to balance out the doses.. in higher mammals.. 1 of the two X's is randomly 99% inactivated. HOWEVER, there remains a small fraction (maybe 1% or more) of genes that are still active ... these genes are necessary to make a woman .. a normal woman. (so, women have 2 doses of a few select genes).

X inactivate occur randomly at a very early point of embryogenesis (probably around the 4-8cell stage). At that point... each individual cell randomly inactivates 1 X. Therefore, some cells will have an active X derived from mom.. and the other cells will have an active X from dad. From this point forward, each "daughter" cell selectively inactivates the same X that was inactive it's mother cell. Thus, you'll havea mosaic of active X's throughout all of the different tissues of the body (populations of cells having active X's from either mom and dad).

So, if the Daddy-cat had Orange fur and the mommy-cat was black fur... you'd have black fur growing from the cells that have mom's activeX.. and orange fur will grow from cells having an active X derived from Dad.

... this is a general explanantion... of course there is more... but i'll keep it simple... unless you need more detail??

take care!
- KIM

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