Undescended testicles are an inherited problem. Testicles are retained OUTSIDE of the body because they need to maintain a lower temperature. When left inside, the retained one often goes cancerous as time passes. They never have one or none, they always have two, but sometimes one or both are retained. If not naturally descended by 16mths, I would have the retained one at least removed. Being inherited, it would be very irresponsible to breed from the dog, it will be passed on.