Natural Science Forum / Biology / Biology / November 2004

Horses and donkeys are different species.  At some point a chromosome
in one lineage broke into two, so one species has n+2 chromosmes, while
the other has n.  Unfortunately, I can't remember which species has the
higher number.

When a horse and donkey cross breed, the resulting zygote has an odd
number of chromosomes.  Since the two species diverged recently, you
still get a functioning animal because the two chromosomes from the
species with the higher diploid number match the single chromosome from
the other species that corresponds to them.

Mules are phenotypically normal males and females, but the problem
arises when they try to produce functional gametes.  Apparently meiosis
gets gummed up, producing aneuploid eggs that can't generate viable
embryos.

Since there are always exceptions in biology, it should be noted that
the occasional female mule has successfully produced a colt.  I don't
think there are any records of a fertile male mule, but this could be
due to the rarity of male mules that aren't gelded.

I may have some of the details wrong here, and hope someone will
correct me if I do. IIRC, there was an article about just this question
in Scientific American many years ago.  Perhaps you can look it up and
tell us what you find out.

Btw, IIRC, horse/zebra crosses are also viable and infertile.