So why cross breed? Are The puppies healthier? NO they are not some breeders may have you believe they are.
When breeding the puppies can have ANY ISSUE that is in the parent lines.
There are SEVERAL genetic issues that are shared by the Shih Tzu and Yorkie.
Here is a list of SOME health issues known in the Shih Tzu and YORKIE Breed.
So when doing a F1(first generation) or F2 or even F3, you need to make sure your breeder is doing genetic testing, so the chance of the parents passing any of these's issues to their off spring is very low to none.
Hetaerists effect and hybrid vigor. Vigor means, "Physical or mental strength, energy, or force." ONLY works with genetic issues that are known in only one of the parent breeds. This does not work with issues in both breed. For these's issues it is like breeding purebred to purebred.
In general we will call the first purebred "purebred-A", and the second "purebred-B".
F1 = 1st generation puppy - 50% purebred-A and 50% purebred-B - for example, a Yorkie to Shih tzu, this is first generation.
F2= 2nd generation puppy - shorkie to shorkie for the first time