All Cosmetics in Gooeys are determined via two individual dice rolls. Your first dice roll determines the potential item rarities you can receive, while the second uniformly selects your cosmetic from the full item pool you've unlocked.
Note that unlocking a higher category pool will unlock all pools below it - for example, unlocking "Mythical" will also unlock both "Rare", and "Common" cosmetics.
Just rolling to unlock the "Rare" pool doesn't mean you'll receive one - this just UNLOCKS Rare cosmetics, and makes it possible to receive items from this category!
Once you have unlocked the pools available to draw items from, your selection roll will then determine which cosmetic your Gooey will receive.
Selection rolls are uniformly random, and simply based off the amount of items available to draw from each pool. Once again, all rolls are performed and secured via Chainlink VRF.
In the event where a rare or mythical body is rolled, rare or better ears must also be rolled! If common ears are rolled, then the body will be re-rolled with the rare pool removed.
In short, a Gooey can have rare ears and a common body - but not a rare body with common ears.
With the above rules all in place, the true rarity rolls for all cosmetic parts can be derived.
In the event where two Gooeys share any traits, there is an 85% chance of their offspring inheriting this trait. If two Gooeys do not share a trait, there is first a 50% roll which determines if a tumbled Gooey inherits a trait from either parent Gooey (with even distribution odds). Following this then, a new trait is selected using the above rulesets.
This is determined on a per-trait basis; for example, if two parent Gooeys share their Mask trait, then there is an 85% chance of their offspring inheriting that Mask trait. However, if they do not share an Ears trait, there is a 50% chance of their offspring inheriting either Gooey's Ears trait - and a 50% chance of them rolling for a new trait.