To me, this dog
looks far more like a natural dog, which can probably function well, mate and give birth naturally, breathe well, and even find it's own prey or defend itself if he had to:
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Not with that coat, he can't. That would be burred- and matted-up in a matter of days. I have seen long haired dogs with mats so tight they can't defecate. A few days ago I was walking Calvin, and came across a little semi-longhaired stray cat with such mats in his armpits that he couldn't stretch out his forelimbs. He had two solid ridges of mats down his hind legs, and everywhere the fur was pulled so tight that his skin was red and puckered, the poor thing. He let me carry him all the way home, in the sweltering heat, and is now cleaned up, neutered, and looking for a new home.
TL, when you mention the KC Spaniel, do you mean the Cavalier or the KC (English Toy)? Are you talking about their heart disease? If you are talking about the CKCS and heart disease, there is nothing to be done about it except for back-crossing. The early-onset mitral valve disease is in every member of the breed, it can't be bred out
Besides that, the onset is such that dogs are usually bred before they have symptoms. Inherited diseases are a lot harder to quash than congenital, for that reason
Of course, any attempt to breed something out of a population means that you're further narrowing the gene pool. It'd be nice if there was a single gene contributing to bad hips, for example, and breeders could knock it out through careful breeding without affecting other genes, but no matter what, you're stuck with a narrowing gene pool.
A friend of mine works closely with a certain breed group, a less common breed, and for a research project he gathered DNA samples of this breed. He needed almost 90 samples before he had 12 *genetically distinct* dogs--meaning, the dogs were so closely related that they did not count as individuals, genetically speaking. Horrifying, right???