First thought:<p>The <i>usual</i> pattern is that only one X chromosome gets transcribed in any given cell.[1]  This keeps the dosage of all those proteins correct.  Otherwise there would need to be a separate set of dosage controls for XX and XY people.<p>The inactivation occurs pretty early in life, and when cells replicate they keep the inactivation.  This results in macroscopic regions of the body with consistent inactivation.  Usually this isn&#x27;t noticeable, but in cats coat pigment is on the X chromosome, and heterozygous cats often show &quot;tortoiseshell&quot; or &quot;calico&quot; coloring.  The patches of contiguous color there are larger than a retina.<p>So it seems entirely likely that all the cells in a human woman&#x27;s retina would use the same X chromosome.<p>On the other hand, that predicts half of het women would be colorblind, which isn&#x27;t observed.  So maybe not so simple...<p>I have a second thought about downstream neural hookups and the mechanisms for those, but it&#x27;ll have to wait for later.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;X-inactivation" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;X-inactivation</a>