Why is inbreeding so bad?
OK. I'm going to hope that you are not talking about people. But for animals it is not necessarily always a bad thing. The problem occurs when too much inbreeding or line-breeding has occurred, and bad genes are present. For example, animal breeders will often resort to inbreeding or line-breeding to enhance a certain trait. This is how come we have so many different breeds of dogs. If you breed a dog with a certain trait to a dog that it's related to, you increase the chances of the offspring having that same trait. Many of the problems that purebred dogs often have is also attributed to the inbreeding that occurred to make them "pure". Think about it, if you breed a dog that has inheritable heart disease with an unrelated dog, the chances of the offspring having heart disease is reduced. But if you breed that same dog with it's parent, sibling, or offspring, then the chances of the resulting offspring having heart disease is increased exponentially. I hope that my answer helps you understand how inbreeding works.