Cryptorchidism — During growth of a male fetus, the testicles develop in the abdominal cavity, pass through an opening in the body wall called the inguinal canal, and descend into the scrotum. Normally both testicles have descended into the scrotum at or shortly after birth. Occasionally descent is not complete until 5 to 6 months of age.
In some individuals, however, one or both testicles fail to descend into the scrotum. Dogs with both testicles undescended are usually sterile, while those with only one testicle undescended are fertile. This is a polygenic (multiple genes involved) inherited defect and when trying to breed out the condition, unilateral crytorchid dogs should not be allow to mate.
Dogs with undescended testicles should be castrated because the condition is thought to be hereditary, the dogs are not eligible for show, and the incidence of testicular tumors in testicles that remain in the abdomen is much greater than the incidence in descended testicles.