I work in the field of Developmental Disabilities, so I've done some minor research on the affects of Downs Syndrome with animals. Downs Syndrome is the only chromosomal defect that is believed to be able to affect all mammals. There have been documented cases of different mammals (horses, dogs, cats) to have Downs Syndrome. The reason is doesn't occur as often as in humans is due to that the mothers body rejects the baby early and they don't survive if this chromosomal defect is present. With that said, I'm not sure I'd believe the theory of boxers being bred with Downs Syndrome. Please correct me, somebody, if I'm wrong.