i have gotten an oniontchi by haveing a marutchi on an even generation i have noticed if you get a marutchi youll get an oniontchi or a hinotamatchi if you have a kinakomotchi you have a better chance of getting young mimitchi or itchigotchi depending on what gen it is on. for the exception of when you first start out you can get a itchigotchi or hinotamatchi but ive always have gotten an oniontchi if i have a marutchi on a even gen.
visit this webpage, quick. then click on conneXion. it shows a tamagotchi connexion growth chart near the bottom. it says how many haerts you have to do (e.g all hearts filled)