Not to come across as sarcastic or caustic, but it has been well documented how "pure bred" dogs have become the way they are through inbreeding to isolate genetic traits. With that trait isolation has come a number of problems due to the homogenizing of the gene pool.
Very true, that's a preamble statement of many genetic studies.
No question that inappropriate breed stock selections will bring forward genetic traits that are undesirable. This link is pretty good overview http://bowlingsite.mcf.com/genetics/inbreeding.html
One of my frustrations is ill informed people referring to a conformation dog show as a beauty contest. Responsible breeders do a tremendous amount of research to breed health stable dog to produce quality puppies.