The way I understand striped and motley, they are two different types of mutation for the same locus on a pair of chromosomes. Therefore, at this locus on each of the paired alleles, you can have either the normal gene (N), the striped gene (s), or the motley gene (m). With these three, the possible combos for this locus on paired alleles are:
NN = normal
Ns or sN = normal het striped
Nm or mN= normal het motley
ss = homo striped
mm = homo motley
ms or sm = motley striped
(het for each, but phenotypically motley striped since both genes are mutated and there is no normal gene overriding them).
If a snake is het for both, it should show pattern mutation somewhere between motley and striped. If you have an offspring from a motley/striped father, then the offspring either received a striped gene OR a motley gene from the father, but not both. If the offspring from this father is a motley/striped individual, than momma gave it the other gene to complete the set.
At least that is the way I have come to understand it.
Oh, and the punnett squares are from Excel, I have one rigged so I just have to type in mom and dad's genotypes and it calculates it out for me. (I like to look at the pretty color combos,

. ) It tends to make more sense to me to see the pattern rather than staring at lists of 1.25% this and 0.0004756% that.
As always, jmho.
I love genetics.