Question time! Because Toby trivia is fun.

A question that a friend at work and I are both curious about: I know you've covered the way children descend from their parents, and the way races descend from the Big Three, but past that -- how do the Firstborn themselves reproduce? If two Firstborn have children, would those children themselves be wholly a separate race, or would they be half of one parent's race and half of the other? Toby is the child of a Firstborn, but since she's also the child of a human, she's still a changeling. And it's come up that the Tuatha de Danann have two Firstborn, each of whom have their own line, and each of those lines have their own scent as far as magic goes, so they can't have reproduced with each other (as far as I can tell the whole inbreeding thing means slightly less to the fae, but if they had then I don't think there would be the two distinct lines with the distinct scents).

Super-sorry if this either is a spoiler for the future or has been explained elsewhere, but we're both super-puzzled, and I wanted to take the opportunity to ask.