Ethical shorkie breeders do all the genetic testing on all their adults. There are many health issues that the yorkie and shih tzu share.
Just because a breeder is having cross bred pups dose not always make them healthier. A cross bred baby can have any issues that is known in breeds it is mixed with.
Here are some of them
Ethical breeders make sure their dogs don't
have any of theses issues before being breed. This can only be done by doing test on them.