Mary didn’t inherit original sin because of the Immaculate Conception.
newadvent.org/cathen/07674d.htm
We declare, pronounce and define that the doctrine which holds that the Blessed Virgin Mary, at the first instant of her conception, by a singular privilege and grace of the Omnipotent God, in virtue of the merits of Jesus Christ, the Saviour of mankind, was preserved immaculate from all stain of original sin, has been revealed by God, and therefore should firmly and constantly be believed by all the faithful.
Now I’m not too sure whether it’s inherited from parents through DNA or other methods, or directly from Adam. Does anyone have an official source on this?
It makes sense to me that Jesus inherited his human nature through the perfect Mary. God already made Mary perfect via the Immaculate Conception, so he didn’t have to do so again for Jesus. Jesus would be perfect anyway, being God and man, but God does like to follow His own laws, and the law could be to inherit original sin through the parents. But I could be wrong on this speculation.