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Old 09-06-2009
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Question X linked diseases in females!

I can't understand why females do not develop X linked recessive disease while they do develop X linked dominant diseases?
In any case they should have only one active X chromosome and the other goes to Barr body, so they should have the disease whether it's recessive or dominant!
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Old 09-06-2009
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Am not an expert in genetics but I think this is the explanation;

Mosaic dominant alleles get expressed while mosaic recessive alleles are less likely to be expressed phenotypically.

In other words, females inactivate one of their X chromosome randomly.

XR = X linked Recessive
XD = X linked Dominant

So when they inherit the XR allele they'll end up roughly having 50% normal X chromosome and 50% XR allele chromosome which is not enough to cause the disease, unless of course they are extremely unlucky when by chance they randomly inactivate most of their normal alleles ending up for example with 90% XR allele and then they may show symptoms (a condition called manifesting heterzygotes). So it's an example of variable expression.

While when they inherit XD allele they end up having 50% active functioning XD alleles which is enough to cause the symptoms. So it's and example of non penetrance (not variable expression).

In the case of males, they only have one X chromosome and it's always active so they'll have the disease whether it's XR or XD.

Why females are more likely to get XD than males? because they have two chances to get the XD allele while males have only one chance.

You may want to have a look at this thread also
Answering Family Pedigree Questions

Last edited by StepTaker; 10-13-2009 at 11:14 AM. Reason: Additions
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Old 09-06-2009
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It seems that you are an expert

Thanks for the explanation am convinced
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