Why do purebred dogs sometimes have genetic defects?

This is mainly due to inbreeding and breeding to extremes for looks and not soundness. For example, the Rhodesian Ridgeback breed standard stipulates that it must have a backwards ridge of hair along its back to be shown, when in fact this is a deformity caused by mild spina biffida. If you are planning on buying a pedigree dog, make sure it has had all relevant screenings and tests for breed specific diseases.