Congenital issues arise not just from the parents, but from the Grandparents and Great-Grandparents.
If one is interested in a particular breed, and knows the breeder, they should be able to make an informed decision. Research the breed and its inherent issues, and make those inquiries to the Breeder.
A reputable Breeder will often "certify" and "guarantee" that their lines do not carry certain known genetic defects, like hips, eyes, heart, etcetera. The guarantee means that if your puppy is inflicted with one of whatever is "guaranteed", you return the puppy and receive a full refund, or a course of action to remedy the situation is negotiated with the Breeder, at the Breeder's expense.
Every toy breed comes with risks. For many of them, nature never intended that they be "that" small. Do your homework, as you are starting to do on here, and gather as much information as you are able. Unfortunately, we do not have many regular contributors to the Forum that have these dogs.
Please share with us what you learn and what you decide.