John Burchard on Mule Birth

Dr. John Burchard puts in a scientific perspective on the mule birth below:

"Not exactly my area of expertise, but ...

"Mule sterility results AFAIK from mismatched chromosome sets. I don't know the
details in this particular case, but in general any considerable difference in
the arrangement of homologous genes on the chromosomes causes problems at
meiosis because the chromosomes can't pair neatly, resulting in non-disjunction
of some chromosomes or parts of chromosomes. That in turn results in aneuploid
gametes or zygotes, most of which are not viable.

"Since in such a case there is a considerable random element in what happens at
meiosis, there may be a remote but non-zero chance of producing a euploid or
nearly euploid gamete, which if paired with a gamete from a non-hybrid parent
might very well produce a viable offspring. It doesn't say who the sire of the
foal was, but presumably a jack or a stallion, not another mule.

"Chromosome incompatibilities seem to result in male sterility much sooner than
in female sterility. I'd have to think a bit about why that is so, but it is
certainly a well-known observed fact. "Bengal" cats are hybrids of Felis
bengalensis (the Asian Leopard Cat) with F. catus the domestic cat. F1 males
are sterile. F1 females are fertile, but must be backcrossed to domestic cats
for at least a couple of generations before you get fertile males. Several
other cat species crosses behave the same way. I think there are also non-cat
examples, though I can't offhand think of them."

I would add that in some falcon hybrids there is often reduced fertility in F1 males.