First intelligence is not necessarily encoded on the X chromosome. But many other things are and that is the problem. To function correctly, cells must maintain a balance of the products that come from their chromosomes. For most chromosomes there are always 2 copies. But when you consider males and females there is an imbalance. Males have only one X chromosome whereas females have 2 and between them they must produce the same amount of product. There are 2 alternatives. On the one hand, the cells of men could up-regulate (double) the production from the X to match what is produced in females on 2 X chromosomes. But the this action could work as well in females and they would have 4x product - unless of course the mechanism is on the Y chromosome. The other alternative is X-inactivation where one of the X chromosomes in females is deactivated leaving 1 X to match males. As it happens, X inactivation is the solution. This happens at the 60 cell stage in embryo development and this is why females are considered genetic mosaics. An excellent example of this is the calico cat. You will never find a male calico - unless it is XXY (Klinefelter's syndrome)
Returning to the problem of an extra "X" either with XXX or XXY - there is some "leakage" and some products are out of balance and this causes problems.
Returning to the problem of an extra "X" either with XXX or XXY - there is some "leakage" and some products are out of balance and this causes problems.