Calculating chromosome number in meiosis: Why is the crossbreed between a fox and a dog (dox) sterile?
This is not simply a question of math, just determining if the diploid number is even or odd. If it was that easy, all hybrids with an even diploid number would be fertile, and that doesn't happen. The problem is way more complex, and it has to do with chromosome pairing.

In the question described, we have the following numbers:


Canis lupus (dog): 2N = 78, therefore N = 39;
Vulpes vulpes (fox): 2N = 38, therefore N = 19;


Now, let's suppose, in an exaggerated simplification, that all 19 chromosomes in the fox's gamete find an exact homologous in the dog's gamete (again, this is very unlikely). So, in this oversimplified scenario, we would have 19 pairs of homologous chromosomes in the hybrid's cells. 

However, even if that's the case, we still have 20 chromosomes in the dog's gamete that didn't find any homologue in the fox's gamete.

That mismatch, alone, is enough to block meiosis, particularly at the pachitene checkpoint.