In the legends King Arthur begets Mordred in ignorant incest. Details vary. Classically Mordred's mother is Morgause, who is either Arthur's maternal aunt or his maternal half-sister. Sometimes - more and more often, in modern retellings - Mordred's mother is Morgan le Fey, Arthur's half-sister; and Mordred is given to Morgause for fostering with Morgause's son Gawaine and his brothers. Morgause is almost always portrayed as an enemy of Arthur, but Morgan is often portrayed more sympathetically and sometimes as Arthur's first and true love despite the obstacle.

When I first started globally replacing the franchises' proper nouns in my STAR TREK/DOCTOR WHO crossovers with proper nouns from the King Arthur legends, I put the revelation that Morgan is Arthur's sister in my story Under Siege. When I began creating notes on the imaginary King Arthur in Time and Space screen canon of which these stories were fanfiction, I realized that this had to mean that the revelation never occurs in the imaginary screen canon. I decided that the imaginary producers must have (like Tennyson) decided that the incest angle wouldn't play for their audience, and had ignored it while (unlike Tennyson) never actually contradicting it. After all, there had been a scene scripted but cut for the end of the STAR TREK episode Who Mourns for Adonis? in which the girl of the week turned up pregnant by the alien of the week; so if unwed motherhood was taboo on '60s tv, then ...

When I began tailoring the imaginary screen version of ENTERPRISE to the needs of my "fanfiction", I needed a reason for the imaginary producers not to use Nimue as a character, since Nimue is analog to the Doctor in my "fanfiction" and there was always the chance that a DOCTOR WHO revival would occasion an imaginary NIMUE revival. So the imaginary producers announced that the theme of their series would be the Morgan-Arthur-Guenevere-Lancelot quadrangle. However, while tv today is more permissive than tv thirty-five years ago, it's probably not permissive enough for the lead's sister to present him with their son (email me if you can contradict me by example). Yet tv today is also more influenced, even pressured, by audiences and by fans who want to know the details. The imaginary producers of EXCALIBUR placed dialog in the third season denying that Arthur and Morgan are blood-relatives.