Hi Abbas,

I use the attached picture as an example: children from a normal father and a disease-carrier mother (with a recessive disease gene). You can see, when a daughter inherit one problem X-chromosome, she can be just a carrier (no disease) because she has another copy of good X chromosome. However, when the son inherit one problem X-chromosome, he will develop disease, because to be a 'son (male)' the other chromosome has to be a Y-chromosome.

Beside, Y-chromosome linked disease can only inherit to sons.