>>Breed him to what? My Red Belly? And the result would be...?
I'd suggest breeding your snake with another Yellowbelly (easier said than done, huh). If all the babies come out with yellow bellies, it would suggest a recessive trait is involved. If they come out with whitish-yellow bellies, then it's probably a co-dominant trait. Of course, all baby erythrogaster have tan bellies when they're born so you'd probably need to raise them up a bit to see the difference in ventral color. Sound like a fun project?
In conclusion, it appears I'm talking out of my ass.
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Pierson Hill

Herpetology and Herpetoculture