I have been nursing a speculation that Henry VI's insanity was not in fact hereditary. It is hard to think that that of his grandfather Charles VI was not, as Charles's mother Joanna of Bourbon, her father Peter I of same and his father Louis I ditto are all reported as being mentally unstable. Four consecutive generations of insanity seems unlikely to be coincidence. However, of the children of Louis I (who as far as I can see was the first sufferer, I can't find that any of his immediate ancestors were afflicted) who were sane, so were their children. Ditto the children of Peter I. And of Joanna. And of Charles VI himself.
With the one exception of his daughter Catherine, mother of course of Henry VI. The fact that the insanity appears never to have skipped a generation before suggests to me that it couldn't; that it was a gene you either inherited or didn't, in the inscrutable process of recombination. And if you didn't inherit it and so were sane yourself, naturally you could not pass it on.