system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does inbreeding produce genetically inferior organisms?

A couple of follow-ups:  
  
In terms of dogs--if you have a German Shepard from one family of dogs and a German Shepard from another family of dogs, would it still be considered inbreeding if they mated?  
  
Are the mutts/crossbreeds of today going to be the purebreds of tomorrow and will the current purebreds eventually die out? (via natural selection because they are genetically inferior?)  
  
Tying into the above question, what exactly is a purebred?  Aren't the purebreds of today mixtures of different dogs from the past? (so, crossbreeds of past "purebreds"?)  I ask because it would seem that natural selection *should* have weeded out genetically inferior inbred's (or purebreds) of the past.  
  
Sorry if I seem all over the place with these questions...
gpt
If there is a detrimental inherited genetic flaw from one half of the DNA, it can be covered up by a dominant version from the other partner. However, if offspring interbreed too much it starts becoming more probable that an offspring will inherit the defect from both parents, resulting in the defect becoming apparent.