Final answer:
In mammals, one of the two X chromosomes in each cell inactivates by condensing into a Barr body, and the genes on the inactive X chromosome are not expressed. This process is random, but all cells descended from a cell with inactivated X chromosome will have the same inactive X chromosome. Mammals do not inactivate an X chromosome.
Step-by-step explanation:
X inactivation is a process that occurs in female mammals, including humans, to compensate for the double genetic dose of the X chromosome. One of the two X chromosomes in each cell inactivates by condensing into a Barr body, and the genes on the inactive X chromosome are not expressed. This process is random, but once inactivation occurs, all cells descended from that cell will have the same inactive X chromosome. Among the given options, mammals do not inactivate an X chromosome.