Glad you find interesting, Satrina :D
I hope you decide to join
When I said "hybrid" I meant a Sim who is very obviously a descendant from an alien. For example because they have alien skin or alien eyes - or some other very noticeable feature, for example if all your aliens are noseless anyone who is noseless as a result will be known to be an hybrid in your town.
The way TS2 genetics work, both alien skin and alien eyes are dominant. So direct offspring from human and alien will ALWAYS look alien.
But they have recessive human genes, and they may pass them onto their offspring - or not.
The chance of passing one or another skin is 50%/50%.
The chance of passing one or other type of eyes is 50%/50%.
Which means that the offspring of hybrid & human have:
25% chance of having both alien traits (skin&eyes) -> hybrid such as his parent
50% chance of having one alien trait (skin or eyes) -> still hybrid, but lower chances on passing their genetics onto their offspring
25% chance of not having any alien trait -> human
And, no, it does not have to be CAS. You may add new adult human Sims however you prefer.
EDITED TO ADD.
Btw I've just finished the second rotation and I'm onto stage 2 :D
Story told here and here, in case anyone wants to read it.