[furball4paws]

Hybrid - the offspring (F1, F2 and/or back cross)resulting from inbreeding between genetically different populations (Futuyma, Evolutionary Biology, 3rd. edition).

Hybrids are often completely sterile, because they fail to produce the appropriate sex organs. A chromosome number mismatch is another mechanism. These mechanisms are very well studied and I suggest you read Futuyma's book, or the book by Niklas referenced in the post). In Helianthus, hybrids of H. annuus and H. petiolarus are almost completely sterile, but there are a few with some fertility. If there weren't there would be no post.

A Chinese and a black African can mix to form a "mixed breed", but since the Chinese and the African are the same species the child is fertile. A mule is a cross between a donkey and a horse producing a hybrid. Except for one report that has not been confirmed, all mules are sterile.

[UnbelievingScumOnTheOtherSide]

Except for one report that has not been confirmed, all mules are sterile.

Those darn exceptions :-)

Anyway, I'm reading about the pollen viability < 10% and seed < 1%. So this is a bit of an exception too.

Another thing you fail to consider is that results in the lab are sometimes analogous to results in a factory. And we all know what to think when we find a pocket watch :-)