Final answer:
The number of X chromosomes is crucial because X chromosome inactivation is random and incomplete, affecting gene expression and leading to possible disorders or sterility when abnormalities in X chromosome numbers occur.
Step-by-step explanation:
The number of X chromosomes matters because X chromosome inactivation is incomplete. Each human female cell inactivates all but one active X chromosome, and the inactive X can express a few genes and must reactivate for the proper maturation of female ovaries.
Although human males and females carry different numbers of the X chromosome, they can function normally because of X inactivation. The inactivation process balances the genetic dosage between males and females. It's a random process, and in the case of females, even though one X chromosome becomes a Barr body, some genes on the inactive X can still be expressed. This inactivation, however, is crucial for normal development, as the absence or abnormal number of X chromosomes is associated with disorders or sterility.