"Mostly" is one of those words that, in alt-universe fiction, means "could easily go either way."
I repeat my prior caveat:In-universe likelihood/genetics/etc is irrelevant; authorial intent is what decides this!
We're told enough to know that there are two factors involved.
1) Genetic predisposition. It's inheritable.
2) There seems to be a "turn-on" element from exposure to magical energy in the womb/early childhood. This was explained with reference to Molly and Charity- Charity was still packing juju when she was pregnant with Molly, but not with Daniel, so Molly's switch was flipped to "on."
This also would explain the "Salic law" tendency of magic inheritance. If mama's got the juju, the fetus is automatically exposed. If Dad does; he needs to be using magic around his pregnant honey to shave the offspring's odds- say moving her around via the Nevernever, which the paranoid Eb might have done.
Maggie was conceived by Susan and Harry after Mama had a bunch of magic tattoos. Will she manifest? Don't know, but with the crowds Susan ran in with the Fellowship and the War it's possible. Potential's likely to be there from Dresden.
EDIT: What's fascinating is that Butcher has drawn a strong correlation between size and power, at least with the Fae. We've discussed before in other threads, I think Maggie Sr spent most of her pregnancy in fast or slow time Nevernever zones to guarantee Harry would be starborn. Her own height aside, I've long wondered if Harry's near-outlier height is an expression of Butcher's Fae correlation.