An  interphase  epithelial  cell  from a
   human   female  shows  a  dark-staining  Barr  body  in  the  nucleus;
   representing an inactivated X chromosome.
   A  comparable  cheek  epithelial cell from a male does not show a Barr
   body.

   Thus,  dosage  compensation  prevents excessive expression of X-linked
   genes in mammals.