ssrpk
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women have 2 X chromosomes ,one of which is inactivated (heterochromatin) which constitues the barr boy! if it is not expressed at all how come women do not express disease with X-linked recessive pattern of inheritance???
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| docofthebigapple
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The Phenomenon is called mosiacism. All women are normal mosiacs when talking about x chromosome. It means there are some cells that shut off one x chromosome and there are other cells that shut off the other x chromosome. When it exactly happems is not clear to me, maybe early in the development as a fetus.
Now lets assign a recessive mutant gene to a X1 chromosome and assume that the X2 has the normal allele. Some cells (i think i should rather say group of cells) in the body turn X1 into a Barr body and some other groups turn X2 into a barr body. The factors controlling this are not known to me. Maybe some other senior members can shed more light to it.
So there is expression of the mutant recessive gene by some cells in the body but it is masked by the expression of the normal gene products by other cells, so we do not see the exact expression of recessive mutant genes.
I'd like to give an example of a cat with black and white skin patches. These are the mosiacs some cells express one gene and the others express the other. If we take one fertilized ovum of this cat and develop somehow 2 cats from it. we would see different skin patterns. What this means is that the process of marking x chromosomes to be barr bodies is random but occus early in development so that a lot of cells divide from that parent cell with a decided barr body. and all express the same X1 or X2 as the barr body, and thus similar gene expressions. hence the black and white patches in our cat here.
I hope this was useful. My knowledge is not perfect and i stant open to corrections. Thank you.
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| ssrpk
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hey man tht was kooooooooool thnx a lot  
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| jiyaji110
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yes thanks for such a useful information.
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| awsab
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that's a very good way to explain..it really clears the concept..
thanks.
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| kingsofke
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just some addons 
Barr body made by DNA methylation but who initiates it dont know.
Mosaicism is an important component of those who survive trisomies like turner patau and edward and even Downs. 99% of turner are aborted those who go to term are mostly mosaic .Anybody remembers association of dysgerminoma with turner well here is the ans why cause mostly are mosaic some turner have even 45X;46XY mosaic that makes them highly susceptibe to gonadoblastoma.
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| CampNou
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X Inactivation is said to be caused by a gene called XIST. X inactivation is associated with DNA methylation (attachment of methyl groups to cytosine base) It occurs early in the female embryo and it is random, fixed, and incomplete. In a chromosome, all cells but one are inactivated. So if it's an XXX female, 2 X chromosomes are inactivated.
Females require 2 copies of the mutation to express the disease since they have 2 X chromosomes. However, in heterozygous females (female carriers who do not express the disease) inactivation of the normal allele will cause the mutant allele to be expressed mildly. These females are termed "Manifesting heterozygotes".
Males are more commonly affected than females because they only have 1 X chromosome, but females are also affected by X-linked recessive diseases, just not that often.
JC
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