If you've been roleplaying in SL long enough, chances are you've seen the lycan vampire witch who also has a demon parent, oh yes and might have some DNA spliced in from another creature to make an uber-mutant. The way our races and rules work, there isn't a good way to manage hybrids fairly (plus they are just silly). The following chart lists the way interracial mating outcomes work in Ascension lore.
Demon x Human = Demon genes dominate, offspring Demon
Vampire x ? = Not possible. Vampires are sterile and can only sire new vampires
Lycan (Female) x Human = Lupus congenitus
Lycan (Male) x Human = Human
Mutant x Mutant = Mutant with combination of parent mutations
Mutant x Human = Mutant OR Human depending on gene expression (I'll spare you the AC Punnett Square)
Hybrids of other races not listed here are not possible.
The Earthborn changelings are full-blooded Earthborn that merely appear completely human until the time of their changing.