Cute snake. 
I don’t work with BPs, though I’m sure someone will come along soon who can give you first hand knowledge about the trait. In the meanwhile, here’s a link to the Morphpedia article about Yellowbelly. There are a lot of sources cited as clickable links at the end of this article, for a deeper dive.
Yellow Belly - Ball Python Traits - Morphpedia&utm_medium=post&utm_content=13&utm_campaign=share-to-facebook