As far as I know, an undershot jaw is a Genetic Defect. I would not breed your Chihuahua if I was you, not only because it will be passed on to the puppies but because it sounds like you have never had the experience of breeding before. Breeding your Chihuahua and not knowing exactly what you are doing, would be putting your Chihuahuas life and her puppies lives at risk. Chihuahuas are known to have complications. You have to know exactly what you are doing, how to spot a complication, how to assist the mother and her puppies during that complication, and you need to know when it's time to take your dog to the vet, for a C-Section. I used to breed Chihuahuas and I've whelped a lot of puppies, but I studied for years and memorized everything, that has to do with breeding before I did bred them and none of my females ever had a complication or a C-Section, but that's only because I knew exactly how to react to any situation and I knew what I was doing. I also had all my males and females Health and Genetic Tested, to make sure they were healthy and right for breeding. When the time comes and your female goes into labor, then begins to whelp (give birth) to her puppies, you don't have time to ask yourself, "well what do I do now", you have to react fast, because if you don't react fast enough, your Chihuahua and her puppies will die.I really don't believe that you would want that to happen! Please, for your dogs sake, have her Spayed. If you want another Chihuahua, go to your Local Shelter or Chihuahua Rescue and adopt one. 
