According to the Mirriam-Webster Dictionary:
Main Entry: 1morph
Pronunciation: 'morf
Function: noun
Etymology: back-formation from morpheme
Date: 1947
2 a : a local population of a species that consists of interbreeding organisms and is distinguishable from other populations by morphology or behavior though capable of interbreeding with them b : a phenotypic variant of a species
Since the granite-backed retics are certainly a phenotypic variant of a species, there is nothing wrong with calling them a morph. At this point, we are determining to what extent they are also a GENOTYPIC variant of a species. Since we've twice been able to produce them from breeding our Axanthic retic to our White Calico, it makes sense to think that there may be more going on there to meet the eye...I mean, it's not exactly like Kevin went through a big bag of WC retic hatchlings, took out all the ones that look the same and pulled a name out of his @$$. He's been doing this long enough to make an educated guess, especially when it comes down to animals that he's produced from selective breeding. 
While we won't know 100% the recipe behind the GB retics until we start breeding them back to each other & the parents, we still have some learning to do with this particular morph, but there's certinaly no harm done in calling them one, especially when you consider that full information & background behind the bloodlines & breeding of these snakes has always been disclosed as well.
Just my $.02
Kara - NERD
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