I am afraid you are going to run into a brick wall without cooperation from the owner (or their heirs) of the litter that your dog was a part of. The registration application must be signed by both the litter owner AND the new puppy owner.
If you are not interested in breeding, I don't understand worrying about registration or lineage. You could call the AKC and tell them your delimma. They may have an idea for you. For instance, if you know the breeders name and the date of birth for your dog's litter, they may be able to track down what dog belonging to the owner, had a litter on that date. I am not sure if they give that information out without proof that you have a littermate. I can't see what it would hurt, if you were just tracking lineage, but I'm not the AKC. Some people and organizations are very private about their records of any kind.
If you truly want to do this, you are probably going to have to contact the heirs to the original litter owner's estate. Perhaps they still have her breeding records and could give you the information over the phone. |