Most purebred dogs are prone to breed-specific diseases, for example, hip dysplasia in Labrador Retrievers.
Mixed-breed (mongrel) dogs are said to be less prone to genetic diseases than purebred dogs due to higher genetic diversity.
I'm going to interpret the genetics portion of the Wikipedia article you cited in plain English, and add what I know from college Biology class:
It depends on the parents you use.
Crossbreeding dogs (hybridization) is a tactic used by breeders to reduce the chance of inheriting a poor trait from a parent. Before breeding, it is important to already know the chance of disease for the particular source breed, and knowing the disease / genetic history of the individual source parent. Genetic testing of the parents prior to breeding may help disqualify parents for crossbreeding.
As a bastardization of theory, inbreeding has traditionally shown to produce poor genetic results; while the introduction of additional genes through cross-breeding may introduce different traits to a line, based on the rules of evolution, the resulting crossbred dog most likely to survive will be the one with the best genes.
