How to make rarity affect baby's species?

[bisexuowl]

the adoptables on my site are going to have a few tiers of rarity:

• Mythical
• Very Rare
• Uncommon
• Common

I want to make it so when two adoptables of different rarities breed, it's much harder to get the more rare species (when you breed a common and uncommon, its a 75% chance to get a common. When you breed a common to a very rare it's a 85% chance to get a common, etc) I already have this rarity system set up thanks to the Mysidia community (!!!), but now I need help implementing the breeding part T_T

I've been looking through the various files and I can't tell exactly how species is being determined (i'm a beginner in PHP), but I think it's in class_breeding.php around line 31?