Greetings from the animal farmer

<...> Ask any pet farmer what results uncontrolled inbriding leads to.<>
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Read the official genealogy of some very aristocratic barbos. You will find out a lot of interesting.

To be fair, the quote you answered was about uncontrollable. Yes, breeders cross sons with mothers and daughters with fathers to get colour (when both are rare), but the unsuccessful offspring either falls asleep or is castrated so that they do not reproduce, because they do not. The animal is likely to get sick.
For people, unfortunately, this is prohibited by law.