I have tried real hard not to say this but just feel it needs to be said. I am certainly not trying to offend anyone but if this a recessive gene that is in both parents and pups, I don't feel that any of them should be bred again. Even if the pups you are producing/selling don't show the signs of the condition, there is a good chance that it will show up again farther down the line. Probably the reason it is showing up more & more is because more standard looking pups are carrying the recessive gene. The only way to stop this is to not breed any of the animals that have produced a blue born pup or were in the litter with a blue born pup. That truly would be breeding to better the Yorkie breed.
I know that is not a good money making avenue but it would be best for the breed. Sorry if I seem cold, I truly am not. I just want the best for the animals affected. Good luck to you.
