First I would like to express to your that I am a Dedicated Shorkie Breeder with babies generally form 3-9lbs full grown.

 
I do several  genetic testing on all my adults. The reason this must be done is to make sure my parents carry no faults for the breed they are. 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 their parent breeds. When you are mixing breeding you are doubling the issues they can have.  Because they are receiving both genetics from their parents.  The whole point of breeding is to better the breed, and to produce the BEST puppies for puppy parents. The ONLY way this can be done it to do testing to make sure your adults do not have any issues they can pass on to the puppies.

For instance a shih tzu can have kidney problems and a yorkie can have Tracheal collapse 

So a shorkie puppy can have both kidney issues and Tracheal collapse..

  Some health issues may not show up until later in life after they are retired from a breeders breeding program. The only way to be 100% that your parent's have no health issues that they could pass on to their off spring is to do genetic testing.  If a breeder has the mom and dad registered no matter what registry it dose not make them Quality.

 All my Shih tzu mom's and Yorkie dads registered with AKC and GENETICALLY tested showing no genetic faults. Any fault the parent has can be passed on to their babies.