Genetic factors predispose metabolic syndrome in mixed races -- ScienceDaily
Info - What is sickle cell disease?
Sickle cell disease is extremely common in Africa and generally speaking people of african descend carry it more so than any other racial group and mixed race babies between any race and an african-descent parent are incredibly likely to get it since again, it's an inherited blood disease, not something you gain, but something you're born with.
And there are a host of other diseases that are more prevalent in every racial group. Whites. Asians. etc. A child of a mixed race breed is likely (not 100% but likely, as in, odds aren't in his favor) to inherit to worst genetic or other disorders from both worlds. So that's why if we'll all be mixed race, we'll all be succeptible to all the genetic disorders known to man. And that is how species go extinct.
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Mixed breed dogs isn't the same as humans. Purebreeds are bread for a specific purpose. Like those dogs that are good at the races. Mix them with another breed and you dumb down their prowess to be that good at the racing thing which is what they're bred to do. You may be making them better in order stuff but that's not what they're there for. They're there to race. It's why it doesn't apply to humans.