Pedigree Analysis - (Feb/12/2010 )

I have a problem which corresponds to the pedigree analysis. There is a disease which is inherited by X-linked recessive. The problem is that the father is affected (XaY) and the mother is homozygote (XAXA). They have 4 children, 2 sons and 2 daughters. The sons are not affected (XAY) and one of the daughter is a carrier (XaXA). The other daughter is not affected. The problem is that she should be affected but she isn´t. What could be the possible cause of that??

X-inactivation - is she homozygous for the mutant?

I have a little knowledge of it,i am so glad to you for sharing this info with us.
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