The YTCA did this survey a couple of years ago.
Health Survey 2007
According to the University of Tennessee, Yorkies are 36 times more likely to be born with a liver shunt than all other breeds combined.
Portosystemic Shunts FAQ
That figure jives with what the YTCA survey found.
The key is to get your puppy from a reputable breeder. Unfortunately, their popularity has made Yorkies a favorite of backyard breeders. You want a breeder who is very knowledgeable about genetics and knows her lines at least five generations back. Make sure your breeder does extensive health testing before breeding the parents as required by the YTCA. The parents should have bile acids tests done, orthopedic exams for luxating patellas, eye, heart exams, thyroid panels, etc. Ask to see the results of all testing down on the parents and don't accept a basic physical as proof of good health.