I never answered this directly. The Gift can be inherited, or it can develop in a myriad of ways. I kind of like the Harry Potter interpretation of magical families to some degree. I would imagine that there are a few in the Order where the Gift passes from parent to child, or skips a generation or two. Or there's some distant familial connection between magi.
Some reasons why magi don't beget more magi:
Distrustful nature of magi, especially their so-called sodales.
The lack of opportunity to engage in procreational sex, for any number of reasons, the first being the Gift being offputting to mundanes.
The relatively small window of opportunity to build a family between gauntlet and beginning the longevity ritual. How many magi actually contemplate trying to go out and find a spouse after they've been apprenticed? Remember, Ra'am was already married before his apprenticeship and I reasoned that the only reason he was able to maintain that relationship was his strong faith, and his teaching ability. In seasons where he wasn't assisting in the lab or being taught, he was busy teaching others, apprentices or otherwise, his Good Teacher offsetting the penalties of the Gift.
Desire to have to deal with a family
When children are born due to a roll in a hay in some faraway land, how would the magus ever find out he has a child? And could that child be discovered, or abandoned, or whatever?
In short, the Gift can be developed in lots of interesting ways, and it is possible that the Gift is there in everyone, but in certain individuals it is just easily developed and brought out. In others, they may never do much or they might actually be squibs.