I think there has to be at least some randomness in how magic manifests because well... Thomas 'can barely cast a tracking spell' Raith and Harry 'Starborn' Dresden are brothers who share a mother.
Yeah, but they both still got something.
If wizards all had strong talents for children I'd expect their to be something closer to dynastic politics in the white council. The senior council seems to mostly be composed of people with independent' backgrounds for lack of a better term, so in canon it's probably not just that.
At the same time, keeping a tradition alive without inheritors across a generation or two is really difficult, especially if no one is writing it down.
For my part the most reasonable middle ground would be if magic always gave you something, but that personal context and various esoteric laws dictated how much you parent passed on. With the most common result being someone with just enough power to stay conversant in magic.
Or rather, the magical traditions that survive long term are those that you can stay minimally skilled in at whatever the average magical inheritance level is to look at it from a different perspective.
For all we know expectant mothers who are magically aware and understand how magical inheritance works do deliberately expose themselves to magic during pregnancy to increase the chances of having a powerfully talented child, and we just don't see it happen because, like with lots of things, Harry is an outsider to magical society and doesn't really know how things work.
That's a good point, though I think it's somewhat implied that it wouldn't be a dominating factor by the lack of pronounced wizard dynasties ruling the council. Not impossible, but if you could get Mortimer/Binder grade sorcerers and wizards with something that direct it seems like it'd have a more visible impact on the setting.
There's also a certain spiritual element to this sort of thing in the Dresden Files, so it might be that power only goes so far without context.
That is, the kid needs something like a significant encounter during the pregnancy, magical breakthrough by their mother early on in their development, or something else significant to kindle power in them by proxy. Similar to how Rosie's talent was kindled by her exposure to movements of power.
That would also play into explaining why specifically Thomas has so little and white court sorcerers seem so rare.
Rather than just blaming it on demons being demons, it would be that the context for the spiritual growth of the kid is already strongly set, so getting something to shift it requires something even more significant than normal.
It'd fit perfectly with the setting because it's the kind of thing that curious wizards could study all they like and still have great difficulty cheesing. Which is something people would almost certainly try to do about five minutes after they notices there's any inheritance involved at all.
