@Carl I've mentioned you because we started this conversation almost a year ago and thought you might be interested in some new info.
My male sim is 25% alien. That is, his maternal grandmother is 100% alien, whereas his mother is a 50% hybrid (child of alien+human union, has lavender skin).
Although my hybrid male sim looks just like a human, he can do a small number of alien socials with other aliens and hybrids.
This male father sim never glows.
His five children with the same human female all look human, but they fall into two categories:
1) One son has been able to do the alien handshake since he was a child. He doesn't glow.
2) One son and three daughters can't do the alien handshake.
He has two children with a 100% alien female:
1) The firstborn daughter can't do the alien handshake.
2) After I got the birth notice, I checked the father's relationship panel and the newborn son appears as a "Known alien." (This is a first for me.)
The father sim went to visit and the baby looks human. When clicking on the bassinette, there was a "He's so...cute?" option. (the baby made weird noises when father clicked that option)
As a child, the only human-like thing about him is his skin tone. He has that alien voice but he can't do the alien handshake. He has alien eyes.
I'm not sure if my game is glitching or if there's more variation in the alien-human spectrum than I first imagined.
I'm playing a dynasty file so I can't switch to the alien household and check if I can change the child's disguise.
Well, maybe I'll make a copy of my dynasty file and see what's going on with this kid.
Anyway, I thought you might find this interesting. It might just be a glitch, lol.