Board Thread:Warhammer 40k General Discussion/@comment-7699236-20130626164907/@comment-27890440-20160229211720

Zavnao - the problem is that although there are a very limited number of fertile cross breeds, they are usually of closely related species. With two unrelated species originating on two unrelated home worlds you don't even have a shared evolutionary heritage. Therefore you don't even have compatible systems of genetic transmission. DNA May vary between earthbound species but it is still DNA, different message, same language. With two alien species there is no common heritage and therefore no common genetic systems.

The only way this could possibly work is if those systems were designed to work together, such as by an ancient progenitor race...oh, yeah, sorry