Our third question comes from tylik, who asked...
"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."
And then parcae asked...
"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)."
Since these questions are basically the same thing, I'm answering them together. Hooray!
First up, let's do a little weird science. It's weird science because it's weird, and because it's not really scientific. At the same time, it is an absolute constant of the Toby universe, so it counts. So:
Oberon, Maeve, and Titania are considered genetically unstable, to use words that we probably all agree mean what I want. Any time any one of them has a child, that child will be biologically unique. Think komodo dragons. Female komodo can (and do) give birth to parthenogenically conceived male offspring who are distinct enough from them to breed with, without worrying about inbreeding. Because nature is weird. So when Oberon and Maeve have a kid, while that kid is definitely theirs, and while both of them would be weirded out and disgusted by the thought of reproduction with said kid (Oberon is not actually Zeus), biologically, they could immediately wait for the kid to hit adulthood and then have kids with their kid, without concern about what that would do to the genetics. Gross, but there you go. There have almost certainly been cases of "Maeve gets frisky with one of Titania's kids, after the kid is a couple of hundred years old," because that are immortal, inhuman, and easily bored.
Any time one of the Three has a child, with anyone, the child will be Firstborn. The genetic instability that the Three bring to the table will mean that the kid is effectively not related to either parent. They'll inherit a class of magic from their Three parent; they may get traits from their other parent (if it is not one of the Three). They may not. Faerie does not like rules, and gets cranky when they are applied. Blind Michael and the Luidaeg, for example, have the same parents (Maeve and Oberon), and do not share any physical or magical traits. Acacia is Titania's daughter by an unknown father, but most people figure Titania didn't bang a tree. The beat goes on.
The one exception to the "nothing from their non-Three parent" is repeated reproduction within a relatively short period of time. If Titania were to have multiple children with the same non-Oberon father during a short span, she would get a cluster of Firstborn who were physically extremely similar, and had complimentary magical gifts. Why? No one knows. One theory is that Faerie is actually guiding the creation of the Firstborn, and doesn't change directions quickly. So Faerie says "I need a type of fae that does _______," puts out the order, and doesn't cancel it fast enough to avoid getting four possibles. This is not a bad thing. Quite a few races have multiple Firsts because of this. (Multiple births can go either way. The Tuatha have two Firsts because the mother of their Firstborn had twins. They were almost, but not quite, identical, and when their children intermarried, the two lines became utterly entangled.)
Firstborn are partially genetically unstable. If Firstborn A has ten children with non-First B, they will all be representatives of the same descendant race. They will also not be biologically related. The DNA gets "scrubbed," for lack of a better term. Fae do have taboos against incest, but only among siblings who actually grew up together. If the same Firstborn goes on to have more children with someone else, whether they get members of the same descendant race, a slightly different descendant race, or a totally new descendant race will be determined partially by the non-Firstborn parent. First + First pairings will always result in consistent descendant races.
Your common scenarios are:
1. Firstborn + member of the Three. Assuming that none of the Three has willingly had children with one of their own children, this still leaves one potential mate for each Firstborn child.
2. Firstborn + Firstborn.
3. Firstborn + other fae.
4. Firstborn + mortal.
5. Firstborn + something else.
Assume the same Firstborn decided, over the course of centuries, to try all five methods. First they breed with one of the Three, and become parent to a descendant race which doesn't thrive. Next they breed with another Firstborn, and become parent to a descendant race that thrives, but in a limited form. Then they breed with another fae, and become parent to a descendant race that spreads and becomes what they are known for. They tarry with a mortal, get a changeling child, and because they cannot find a hope chest, go no further. Finally, they breed with a cloud, and become a parent to a second well-established descendant race.
Faerie is complicated, and Mother's Day cards are confusing.