Striped in the California king snake is caused by a dominant mutant gene. I'm not certain of any others.

As for why dominant and codominant mutants are more common in boas and pythons, I think it is because the commonly kept colubrids are more active. They get out and move around when seeking food, while the more heavy-bodied boids are more likely to be concealed in order to ambush prey. In other words, a snake that spends 10% of its time in the open is more likely to be spotted by a predator than a snake that spends 1% of its time in the open, all other things being equal. And if a mutant is somehow a disadvantage to survival, which most are, more or less, then the very first possessor of a dominant mutant gene is more likely to be weeded out of the population than the first possessor of a recessive mutant gene.

Paul Hollander