The key is that <i>which</i> X chromosome is deactivated is not the same in all cells.<p>It does occurs fairly early in the development process (a stage called gastrulation), and the deactivation does persist for all cells that descend from a given cell present. But critically, it&#x27;s late enough that a retina could conceivably possess cells descending from two different &quot;lineages&quot;.<p>You can actually visibly see the &quot;resolution&quot; of the deactivation by looking at tortoiseshell cats. Each blotch of orange&#x2F;black represents one cell present at the deactivation stage.