I thought it was impossible to pass freckles genetically in-game. So I always added them manually. But then I saw it happen ONCE in all my thousand of hours of playing. I was ecstatic.
This is the Mom:
https://i.imgur.com/ZnOc5qo.jpg
This is her son:
https://i.imgur.com/VrYSyEm.jpg
He came out looking like this. So the freckles were there the instant he aged up into a toddler
https://i.imgur.com/VMb6iBD.jpg
So... it's possible to pass them on genetically in-game, but I'd argue it's the rarest event I've ever seen. You'd be better off adding them manually.