Besides what other response have said, the X chromosome passed from a mother to an offspring is not typically identical to either of the X chromosomes in her ow

Besides what other response have said, the X chromosome passed from a mother to an offspring is not typically identical to either of the X chromosomes in her own body. Rather, the X chromosome passed on through her eggs is a blend of her two X chromosomes. This process is called genetic recombination and is basically what allows sexual reproduction to roll new genes. Since men (typically) only have one X chromosome to start, the X passed in their sperm is identical to that of their somatic cells. So two sisters with the same parents will share one identical X from their father, but the second X from their mother will be different combinations of her two X's. Edit to add: if genetic recombination didn't happen, then lots of siblings would end up as genetic twins. (50% chance within the same gender). And that would also be identical to the offspring from one crossing one grandparent on each side... Fortunately this doesn't happen!