OK, it has been a long time since I kept any Ratsnakes, so I had to do some searching, and I have found - as I am sure you have as well - that Google is NO help here!! So, I went to Kingsnake. I found a thread discussing this morph, and I thought it was interesting: http://forums.kingsnake.com/view.php?id=1602976,1602976
The short answer would be that the morph appears to be Simple Recessive.
This means that a white-sided black rat X normal black rat would give you 100% normal-looking black ratsnakes heterozygous ("het") for white sides.
That also means that a white-sided black rat X het white side black rat would give you 50% white-sided black rat offspring and 50% het white side (that's 50% of the offspring, but KNOWN hets, aka "100%" hets)
Also, het white-side X het white side would give you {statistically} 25% white-sided, 50% het, and 25% normal - but since you cannot visually detect which normal-looking offspring are hets and which are truly just normal, these would all get labeled "66% Possible" hets (as 2/3, or 66% of the normal-looking offspring can be expected to carry the white-sided allele) - and in my opinion, that is too much of a marketing headache. :) Many people disagree with me there, but I LIKE the morphs myself, and have bought the homozygous recessive to enjoy for myself and breed to hets - I like knowing exactly what I am producing (or GOING TO produce anyhow, *sigh*). I heard a breeder comment once, and I can only heartily agree: "life is too short for possible [hets]." They can be good to buy if you don't mind that you could end up with a normal instead of a het. Anyhow, my unsolicited 2 cents there. :)
I hope this helps. :)