Question time! Because Toby trivia is fun.

How does lineage through firstborns work?

I mean, it seems like it's still sexual reproduction, and sometimes both parents are firstborns, but it seems that descent of record is usually tracked only through one (so, maybe dominant descent? Or are their multiple varieties of offspring any given firstborn may have?) But in at least one case there are multiple firstborn, and while firstborns clearly have children by people who aren't firstborn themselves, logic suggests that at some point the mating population was only other firstborn, their parents, and Something Else Altogether.

(All entirely understand if the answers to this are too plot relevant. But it seems equally likely that at least parts of it are one of those things that everyone knows, except, y'know, us readers.)

Kind of related to this: I understand that you get a new race when two Firstborn have a kid, like when Blind Michael and Acacia got together and had a Blodynbryd. But will two Firstborn always produce the same kind of kid? By that I mean: if Acacia and Blind Michael had had another kid, would it invariably have been a Blodynbryd? Or would a second child have been some different new race?