Different sources are vague - all Lexicanum says on the subject is that the gene only breeds true between two Navigators. Considering even the severest abhumans like Ratlings and Ogryns can successfully cross-breed with humans and make hybrids of sorts, not to mention outright Xenos pairings, and all the other weird stuff that the gene does, it seemed entirely possible that there might be more issues than just the lack of passing on the gene.