my gsd had a retained testicle. which was discovered by our vet. at a good few months of prodding and wondering. eventually it was taken away. and he was neutered. should we have informed kennel club. breeder was registered, does it mean all males had it.in the pack.my dog is lovely and happy, but surely there must have been a dodgy gene. how common is this. it was awful for him and costly for us. we wouldnt change him for the world.do you think the breeder knew. he did saythey would have a future litter. will they be affected.we had right papers and all was in order.until this.i have always wondered.dog is two now.

It is more common than some like to admit. It is down to the genes, but NOT all in the litter may have been affected. The breeder may (should have) been aware that it was not there at the time you purchased him, but it is also common for the testicles to drop later.
I have a Beardie who retained both testicles, but I knew at the time that they were not present, I "knew" by feeling, but the breeder also pointed it out quite clearly (I had him castrated at 18 months old, and the breeders paid half the cost, not coz I asked, but they felt so awful that they had not come down).
Some dogs testicles are not fully down at 8 weeks of age, in some dogs, they come down a little later, some don't come down at all.

Kay (Kayanarra)

It would be a courtesy to tell the breeder so they can keep an eye open in the future, you did not need to have him castrated you could have just had the retained one removed and left the other. You only need to tell the KC if you wish to show him and I guess that you don't or you would have started when he was a puppy.