In REALLY plain (probably grossly oversimplified) English:
"a" (or "an") = "not having any"
-xanth = yellow
:)
(In other words, the snake is lacking yellow pigments.)
What causes it? Most likely, a defect in one of the enzymes responsible for the synthesis of that yellow pigment, caused by a mutation in the gene that codes for that enzyme.
Is this what you see in "lesser" morphs? No, unless I'm completely misunderstanding your question. "Lesser" is its own morph completely separate from axanthic. A nice lesser will often have a LOT of yellow pigment. ;)
