Tiny correction, Nanci. Stripe and motley are two of the three alleles (the third being the "normal" allele at this locus) that reside at the same locus.
To answer the OP, stripe is a genetic mutation, but the "vanishing" phenotype of that genotype can be thought of like a line-bred thing. And, as Nanci said, apparently cubes are, genetically, stripes--another phenotypic expression of the same gene. (See here? I should have said "another phenotypic expression of the same allele. Everyone does it!)
The same is true with your motley whose pattern is stripey. Pinstriping can be thought of as a line-bred expression of the motley gene. (Should read allele.)
Both the stripe gene and the motley gene (alleles) seem to have a great deal of variation in phenotypic expression. Where a particular individual that possesses those genes (alleles) lies on that phenotypic spectrum is dependent on other, as-of-yet-unknown factors (probably other genes, but who knows).
One thing that hasn't been mentioned yet, though, is that motley is dominant to stripe.
There are some nice explanations of this by Chuck Pritzel on Chuck's and Connie's forum.
Reason for Editing: Edited to correct places where I used "gene" when I should have used "allele." Lots of times, we use "gene" and "allele" interchangeably, but really that's just being a little lazy. There is nothing that is interchangeable with "locus," though, even in shorthand.
