As of right now, here is the way it works:
- Any follower is born Common/Blue (75%), Uncommon/Yellow (20%), or Epic/Green (5%), whatever rarity their parents are, whether you use the Fertility power or not
- For any two followers to pair, there has to be:
- At least one male follower in the village
- A female follower who is not working and not pregnant already
- Plus, the more followers in your village, the less likely every female follower is to get pregnant
The first point regarding rarity is going to change in next version 1.8, which should be out in ten days or so.
Details will be provided once this happens, but the rough idea is that probabilities should then depend on parents' rarity, with Epic parents more likely to give birth to an Epic child, and the Fertility power equivalent to a Common father.