Father-daughter and mother-son breeding... I would like to know what the big deal is...
The two previous answerers failed to check the category: Pets & Animals.
The answer is because each time an animal is born it inherits both the good and bad mutations that exist in its parents. By inbreeding you double the chances of the negative mutations becoming predominant in the offspring. This is what causes species to become extinct -- they get locked into a confined area and start inbreeding until the negative mutations cause them to die off.
That's why zoos bring in animals temporarily from other zoos to breed -- to prevent the blood lines from becoming weak.
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well its really that when you are born you get trait's from your parents and some can lead to physical and mental deformities. but they don't have enough of a dominant effect. but since you and your parent has the same trait you double the chance of having your children with a deformity