It's a single gene, not a hybrid. We know this because the offspring do not look like gradations in between both parents--they are either deserts, or they are not. If the females are not deserts, then they are not sterile, even if they had a desert sire.

In hybrids, the offspring will look like a cross between the parent species--they may look more like one than the other. You will often get individuals that look more like one parent, and others that look like the other parent, in the same clutch/litter. Deserts look like ball pythons, and not like anything else.