Most hybrids are sterile and can't reproduce. Cross Stripers and Sand Bass and you get the hybrids that are stocked in many reservoirs, but these fish cannot reproduce. Mules are sterile hybrids from horses and donkeys. Hybrids do occur in the wild but there are usually reproductive problems associated with them that do not allow them to breed further and the genetics become part of the makeup of the species.

As for mulie/whitetail hybrids it is nearly always the much more aggressive whitetail buck breeding the more non-agressive mulie doe, and not the other way around. Also true mulie/whitetail hybrids as provable by genetic testing are not as common as some biologists and "experts" would have you believe. Really the main issue with these species is that whitetails are more agressive and just flat out out-compete their larger cousins. But that is a loooonnnng discussion.