
Originally Posted by
Clirkus
It happens a lot in the dog breeding world, because there are so few "perfect" examples of a breed, father dogs will often be bred to their female offspring, brothers to sisters. There are certain rules in place now but it still happens. The gene pools of pure-bred dogs have become so shallow that it causes a lot of genetic diseases and weakened immune systems to happen, which is why mutts are generally healthier than dogs with a pedigree.
There's actually a kind a spaniel that has a common genetic disease that literally causes the animal's skull to be too small for it's brain, and it's very painful. Yet, if a dog with that condition has all the other right qualifications for its breed, it could go on to sire dozens of litters and pass it on.