Legacy of th Gift

All the Magi in my saga want a child (or more...).
Now, the question is:
Virtue and Flaws like Mithic blood are ereditary? to all the children or only to the firt born.....

I think this is something for a GM or troupe to decide.

Looking at House Mercere which has a strong family lineage, one can see that the Gift is not gauranteed to manifest in every generation. But then, this may only be the case in this particular family. Perhaps like the Skywalkers, every generation has a more powerful link to the force/magic/whatever. Or it skips a generation, or only the first born girl,or second born son, or whatever.

I think you should figure out the players want, then tweak it. Tweaking it makes it interesting. So if they want a male heir to carry on the good name, give them only girls. Or a frail boy, or something to make it more interesting than their dream come true.

True lineages kind of suggests that magic is passed down the family line....

IIRC, it doesn't.

It does suggest that a number of the blood descendents of Mercere are Gifted, but doesn't give any indication as to whether this is an unusually high (or low) number of Gifted descendents.

I think it's about a dozen(?) Gifted descendents after c. 400 years, with a lifespan of over a century. Given the cult of personality around Mercere and the House's needs, I would suggest a higher-than-average number of children per generation, and magic used to ensure that more reached maturity.

Assuming a (reproductive) generation of 30 years, we should be on to the 14th Generation. Even if we assume an average of only two adults reached maturity in each family, that should be 16,384 children in the most recent generation. Three generations will be active (100 year lifespan). So around a dozen magi in approximately 28,672. ~0.04%

I think all the assumptions are too low.

The "Lineages" referred to are magical - Parens and Filius/Filia.