What is it?

From time to time a male pup is born that doesn't develop like ordinary males do: one or both testicles decide to stay inside the body instead of dropping down into their appropriate pouch. Traditionally this is seen as a very grave affliction. Every breed has a line in its standard that afflicted males are banned from showing. This gives rise to the thought that they should also be banned from breeding. Most breeders stick to that. When both testicles are stuck inside there is no other option, because the dog will be sterile. With just one, it is a different matter. He might be a good sire or he might be sterile. And to be complete some dogs just have one testicle, the second just isn't there (i.e. it is not locked inside the body, but completely missing).
With one testicle in it's pouch the affliction is called 'monorchism', when no testicles appear 'cryptorchism'. Sometimes both forms are addressed as cryptorchism.

Is it hereditary?

Now this is something that is not appreciated so the question is, is it hereditary? If so, not breeding the dog is the logical option, as you don't want to breed something bad into your line.
Traditionally vets proclaim it to be hereditary and dangerous. Well now, they would, castrating dogs is one of the main sources of income for a vet and proclaimed often as thé remedy for almost any so called problem you come across. The fact that castration is a real operation and therefore quite dangerous in itself, is often neglected or slightened.
Fact is that there has been no research to this phenomenon. Simple because it is very difficult: you have to have a reasonable number of dogs with the affliction, you have to monitor them during a long time (10 to 16 years) and you have to know their background. Non-pedigree dogs won't do because mostly their parentage is not known. And where will you find people willing to cooperate to something that for ages has been proclaimed to be dangerous for their beloved pet? Especially since their is such a "simple" solution as castration, and knowing castration "eases" the traditionally "harder-to-handle" male dog?
The theory is that it is warmer inside the body than in the pouch and that would give rise to cancer. This might be so, but one wonders why testicletumors take so long to develop and the problem doesn't rise in young dogs.
In the mean time I have taken the time to do some research myself. Not in dogs, though I have seen this affliction a couple of times and in non of these cases they got - or worse died of - cancer. One cryptochide dog died at the age of six of a virus, a friend of mine had a monorchide dog that lived to the ripe old age of fourteen and died of kidneyfailure, two others are still alive, the one a very fertile father of nine pups (5 males, none afflicted) a non-pedigreedog, the other non-pedigree and recently neutered.
So I have searched for information amongst our human kind. Simply because men are being studied and people generally don't see castration as an option for a human child, so we have material and it is being studied for a longer period of time. In people doctors are not sure if there is a genetic link and mostly men afflicted are either sterile or produce normal offspring. They do see a genetic link when it comes to testiclecancer, but to their experience it is not so much the monorchism that is hereditary, but the tendency to get cancer. The warmer environment-theory is disproven, because in humans when the testicle that is stuck inside is operationally brought down to the pouch, it still has a greater chance of testiclecancer than the one that was outside in the first place. People with a higher chance of testiclecancer are also prone to other forms of cancer and it might be that the hereditary-cancer-chance comes from the side of the mother! (nothing to do with the testes, but a genetic tendency to get cancer in general).
That being warmer is bad for the seed on the other hand is a true and proven thing and that is why cryptorchide males are infertile, as well as some monorchide.
Back to dogs: since there are bloodlines that have a tendency to produce quite some dogs with this affliction, for example Bull Terriers, you might think it hereditary after all. In the case of Bull Terriers, there is an important sire that practically 'build' the breed and he was a monorchide ...
He also had some other distinctive features like the roman nose. Now some people believe that the fact whether or not the testes drop into the pouch is not decided by genetics, but by the later development. The build of the male in question being an important factor whether or not this happens the proper way. Just like siamese twins are a fault in development during the pregnancy, not a genetic disorder conceived at conception. (An example that I have seen on Discovery that got stuck in my mind). Since the testes drop after birth - which in dogs can happen anywhere between 6 weeks and 9 months - this is a very alluring theory. Now back to Bull Terriers: they have a very distinctive build, with characteristics that differ from almost all breeds. Indeed there is one male in particular responsible for these characteristics and he was monorchide. Traditionally this is seen as evidence for a genetic link. But there is another line of thought possible:
for ages the males that are afflicted have been neutered, so they have no genetic part in the breed, still the affliction exists. Sure it might be a recessive gen, but when in any breed one characteristic is so carefully bred out of it, you might have a lot of trouble keeping that characteristic in the breed when you decide otherwise. Take for example the colour 'harlekin' in the Beauceron. For a long time this colour wasn't appreciated and no one took any specific action to keep the colour in the breed. On the other hand, no action was taken to eliminate the colour. The result is that nowadays breeders that try to keep all the accepted colours in the breed have a very hard time keeping 'harlekin' alive - and that is by breeding the dogs that have this colour: there is no other way to be sure this colour will still be here in another twenty years or so.
Think about it: there has been no action to keep 'harlekin' actively out of the breed and still it is almost extinct. Monorchism has been banned for decades, even more sure you can be about cryptorchism, but these afflictions still exist.
The other option is that the particular build of the Bull Terrier has a genetic tendency to be so different from natures thought of a canine that testes have a hard time to find the pouch. In other words: their development is hampered by their build and that is why this affliction is seen so relatively often in Bull Terriers. Taking this to the level of all dogs: sometimes the development of the dog is hampered for some reason or the other and that is why he only has one testicle or none in the pouch.

The effects for the afflicted dog

The other factor to consider is: is it a problem for the male that is afflicted. If it is not hereditary you might even breed such a dog, but what about the dog itself. Is he in danger himself and if so, how great are the risks and what can the owner expect. I mean: when someone takes in a dog like this he'll want to love it just like any other dog and not be worried sick.
As stated before there is a chance of cancer, but as far as doctors can see it is the other way round: a bloodline with a tendency to cancer has a greater chance of getting testiclecancer, when a male is monorchide or cryptorchide. How big this chance is depends on who you ask: vets proclaim testiclecancer a high risk, just as uteruscancer and teatcancer in bitches (I probably haven't used the right english terms, but you catch my drift. If you can give me the proper names, please do, I like to be accurate), proclaiming spaying and neutering as the ultimate solution (I'll get back on the female part at the end of this article). Doctors who work with humans are a lot more careful and say the chances of testiclecancer are slim in any case and though bigger when a man is monorchide / cryptorchide, still it is a very slim chance. I think the truth lies somewhere in the middle: vets want to castrate to earn their money ánd because they also realise there are quite some dogs in the world, no need to produce more just to add to these numbers (look at the shelters crammed with lovely homeseeking pets); doctors want to reassure their patients and since in humans the ball inside is mostly brought down into the pouch, cancer can be easily ánd early diagnosed. In my personal experience the chance is slim indeed for I have seen several cases and none had / has cancer, while on the other hand I have had to let my Beauceron go for prostate cancer at the age of eight years, while he was a very healthy complete male out of a very healthy working bloodline. Of course there were people saying afterwards 'you should have had him castrated, then it wouldn't have happened'. Perhaps not, but he also wouldn't have been the wonderful proud dog I have known him to be. And a Belgian breeder lost a perfect Beaceron at the age of 18 months, because he had to be put under a light anesthetic ("just a couple of pricks, nothing major" vets tend to say) when they took x-rays to check his hips. At least my dog had never been at the vet for any other reason than his yearly vaccinations. There is no telling when you'll get hit by disaster, only when it does, it always a hard blow. Big chance or small.

So what are the options:
As seen before some proclaim castration as a good solution. Also because some claim that when the dog does get cancer, he might get what is called 'the feminisation syndrome', that is physically he becomes a female. Well, I have once had a Dobermann from a shelter and that was a castrate. Since he had been castrated at an early age, he grew out to be a 'normal'dog, but he never became an adult dog in his mind. His other problem was that he got mounted by just about any male around and quite severely attacked by females especially when they were in season, because he smelled like a bitch in heat as my vet explained me, because he had been castrated and that sometimes happens ....
I won't argue that you shouldn't do something about cancer, but preventive action can sometimes be a real problem in itself. You have to weigh the chance of the one against the other: what is your greater fear, will the dog develop cancer and if so won't that be treatable, or will the dog not be himself, even lose his complete position in dog-society (as my Dobermann had, I admit it doesn't happen often, but when you're stuck with it, it's a damned nuisance) or even die of the anesthetic, risks all taken as a preventive action for something with a very small risk-margin itself! Not to mention the fact that the operation to find a 'lost' testicle is a far worse one that a normal castration. I recently saw an operation of this kind on television, where the search for the lost testicle took almost two and a half hours!!! And there is always the possibility that there is no second testicle ... (I wonder how long the vet will keep searching). Don't be mislead by rumors of laparoscopic surgery: yes, it makes it easier - when the testicle wasn't hard to find in the first place - but most vets don't have all this new equipment (I know mine hasn't). And this kind of surgery is also promoted to spay bitches, but there the vets that have this new equipment are behind in other respects. Recent experience has given rise to the notion that just removing the ovaries and not the uterus does not diminish the chances of female types of cancer and in fact might even enlarge it! (Source: a documentary on Discovery, unfortunately I can't be any more specific, but since they have reruns of just about everything I'll probably will be able to mend that in time).

The conclusion

To summon it up: I myself would never neuter a dog for this reason, since the risks in surgery in general are in my view higher than the risk of cancer related to this affliction. Other forms of cancer related to a male being a male or a female a female can only be bred out of a breed when we know it is there in the first place, so I am against castration anyway.
Does this mean you have to let your dog get ill? No, of course not!!! But you have to keep a close eye on your pet, whether he is special in any way or 'regular' in conformity with the laws of nature. Just a couple of decades ago dogs didn't grow to be as old as they do now for all the good care they are given. That also means there are more forms of illness that can appear: any living creature has to die of something (unfortunately ...) and dogs just don't live as long as humans (extremely unfortunate).
When the dog becomes a veteran (at 6 or 7 years - the time a century ago most dogs died) you have to be more alert.
When there is cancer in the bloodline, castration might be a wise choice, since the risk-margin in that case is far higher.
When you suspect the dog is not allright or you have a bad feeling, consult your vet. An extra trip to the vet never hurt anyone, waiting too long does!
What breed of dog you have makes a difference too: I don't have statistics but to my experience Dobermanns and Rottweilers have a higher chance of developing cancer. During the time I had my shepard a woman in the neighbourhood has had 4 Dobermanns who died of bonecancer. I must admit that the way she trains her dogs might also have something to do with it, but a dog from that breed will definitely have a higher risk-margin for testiclecancer, if we go by the theory that bloodlines with cancer also have a higher risk of developing testiclecancer when they are monorchide or cryptorchide. Also breeds with a higher cancerlevel influence the percentages of cancer in canines. A breed like the Lakeland Terrier is known for the fact that it is a healthy breed (an important reason for me to choose this breed as my breed, as well as a general long life expectancy), with very little or no occurrence of cancer. Also a reason to believe monorchism might not be a problem in this breed. Still it is very difficult to say something about a breed in general, mostly because of the silence breeders keep. So far I have never met anyone with a pedigree monorchide dog - any that were have been castrated and people don't talk about it. My first dog was a cryptorchide and a pedigree dog (a Yorkshire Terrier), but we never knew that was anything peculiar. The breeder kept very quiet about it! He never had a problem on that account, but died at the age of six as mentioned before. The suspicion was that it was a virus - we were on holiday at the time and there was no autopsy - but it certainly had nothing to do with his testicles.
Anyway depending on the breed you choose there are specific troubles: big dogs have a tendency to get bloat. I never had the problem with either the Dobermann or the Beauceron, but I always had the strain of making sure the dog didn't run or play wildly after eating, which was particularly difficult on busy days (sometimes you just want to feed the dog, give it a run and than hurry off to work). With a Dobermann you always have to keep an eye out for changes in behaviour since they might develop tumors in the brain, which can lead to abnormal aggression, Rottweilers also have this tendency.
Very small dogs can get problems with open skulls (watch out for sudden bumps) and misplaced teeth, that make eating difficult and in old age even impossible. But I am straying.
The problem you have to watch out for is a hereditary chance of cancer. Not all forms of cancer are hereditary, some are a result of an inappropriate lifestyle. When there is hereditary cancer in the bloodline, you wouldn't want to breed a monorchide male out of that line and castration would be a wise precaution. I would go even further than that: I don't want to breed any dog that has a known chance to get cancer - in whatever from and whether he has one, two or six testicles! When there is no enlarged chance of hereditary cancer in the bloodline (I think it is impossible to ever be 100% sure there is absolutely no chance of cancer), there is no reason not to use the dog, other than the fact that he must have all the qualities you would demand of any dog that is going to be a stud - like excellent character, good build, nice coat, good set of teeth etc. and suitable for the bitch you want to use. All of which makes it very hard to come through my selection indeed! Though one must keep in mind no dog is utterly perfect. One may look good, behave perfectly, even produce wonderful offspring, but there can always be a hidden defect. Some defects are unimportant and do nothing to diminish the breed, like the tendency to get a white chestmarking in one colour dogs. Others are far more intrusive, like hereditary liverdiseases, that only come out when the dog is at a veteranage (as in the Bedlington Terrier, though that has been quite succesfully dealt with). This is also why it is not a good idea to use a very prominent showdog with excellent results too often. It is better to keep a wider range so the hidden problems will be divers and not the same in a lot of bloodlines. Also it is good to remember that inbreeding is the most common reason for cancer: hence the high rates in small pets like rats and mice.

Epilogue

The fact that no dog is perfect is certainly taken into account when one is breeding dogs - or anything else for that matter. As a (good) breeder you know the shortcomings of your dog and you try to find a mate that will hopefully alleviate these shortcomings. Of course the mate has shortcomings of his own and that is why you have champions and simple pets in the same litter. Some pups have the good qualities of both parent (what every breeder hopes for), some have some good qualities and one or two shortcomings, and some have the shortcomings of both parents (what every breeder tries to avoid, but there is no accounting for nature). You try to select the best pup(s), but growing up takes its time and most pups leave at the age of 8 or 9 weeks - that is why sometimes a breeder has to sell an older pup (a shortcoming appeared only later in his development) or a petowner has this beautiful dog, that didn't look like much as a small pup.
I personally attempt to avoid these mistakes by asking prospective owners to allow me to use a male pup as a stud later on: whether I'll actually do so depends on his further development. Unfortunately it is far more difficult when it comes to the female counterparts. When I sell a bitchpuppy I leave it to the owner whether he wants to breed her or not. I don't think it is fair to put people up with the trouble of a litter (they often think it will be fun, but it is hard work and it goes with a great responsibility) or taking the bitch back when she has to whelp and afterwards handing her back to her owner. At such a time she needs the person she trusts most - as she has grown up elsewhere, that won't be me. And whelping changes a bitch, the owner might be surprised at what he gets back.

Mind you, at the beginning I told you there was no scientific evidence, but this is NOT a scientific survey either. I can't summon all the sources I have used for this is information I have gathered during the past twenty years or so, added with some information I got from "Korrelatie" and Discovery Network. I just wanted to show that there is no scientific evidence to assume monorchism is dangerous or hereditary. You can argue to the other side just as easily: there is no scientific evidence it isn't (though I certainly won't support that view).