I think if you do not neuter the internal testicle, it is possible that it will twist and cause pain, possible infection or worse. But, like everything else it is just a risk. There are some that do not believe that cancer is more likely in a retained testicle, but spotting it would be near impossible before it is too late, and since, this cancer actually does have a good prognosis if caught in time. That would be sad. Again a risk you take.
I know of a monorchid dog that sired 10 male pups (2 bitches) and all of the pups had both testicles by 7 or 10 weeks. I would imagine that the pups would all be carriers. Unless there are actually more reasons for the problem.
I mean, if it is genetic if the accompanying hardware was too short to descend properly, or the testicle(s) were completely non-existent, then all the pups would be carriers of that condition. If it were more the descending and ascending testicle, and the ring closed when it was on the up-swing, maybe that isn't a genetic issue? And then puppies out of that dog might not be affected at all.
But I really do not know.
It is a pain for owners because of the extra cost in neutering, and because the dog cannot be shown. The dogs are certainly able to sire puppies, perform in dog sports, and make as good pets as their brothers that have their equipment properly descended.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.