MadSci Network: Genetics

Re: Can hinnies or mules produce offsprings with donkeys or horses?

Date: Sat Sep 20 21:29:40 2003
Posted By: Greta Hardin, Secondary School Teacher, Science
Area of science: Genetics
ID: 1062450750.Ge

The answer is, "No... usually."

Hinnies and Mules (almost) always have the appearance of females, but are 
(nearly always) 
sterile.  The reason for this is due to the complex hormone signaling that goes 
on during 
fetal development that determnes whether an organism is male or female.  In 
when the signalling is messed up, or not there, the "default setting" (or what 
organisms go 
to when no other information is available) is female, but sterile.  In order 
for a mammal to 
turn out a functional male, a predetermined level of androgens (and other 
hormones) need 
to be there.  If they are not, or in the wrong amounts, or at the wrong times, 
the signalling 
does not work, and the fetus cannot develop correctly, and usually turns out to 
female, and sterile.  Similarly, for a fertile female to develop the correct 
timing and amounts 
of hormones are also needed, and again, when things are out of kilter, the 
fetus will turn 
out female in appearance, but sterile.

Now why does this happen in hinnies in mules?  Well in each, you have an 
embryo/fetus that 
has half the timing and receptors of a horse, and half of a donkey, and are in 
a whole horse 
(or donkey) mother.  So the timing and the amounts of hormones are going to be 
determined by the horse genes, half by the donkey genes and getting cooked away 
in a half 
alien mother - so the timing and the amounts of hormones thus available will be 
wrong for 
the little developing hybrid.  Thus all you will get are very sturdy animals 
that are female in 
appearance but sterile - their reproductive organs never developed correctly 
because they 
never got the right hormones in the right amount at the right times for them to 
work - well 
- right.

Now, you may be asking, why do I have the disclaimers at the top (the "almost" 
and "nearly 
always")?  Well, occasionally there are exceptions.  I do remember a story when 
I was in 
grade school of a mule having a baby... ummm, well they called it a mule, 
though the father 
was a horse (what else would one call it, I guess?).  All this proved however, 
is that there is 
wide variation within a successful range.  Somehow, during that particular 
mule's gestation 
it received the right amounts of hormones at the right times to develop 
reprductive organs.  And knowing this, I would be a fool to say equivocally 
that somewhere 
there are not mules that appear to be males, and possibly a fertile one out 
there as well.

Finally - due to the "apparent" sex of mules, looking at the chromosomes to 
"true" sex, while a possibly interesting exercise, is not really useful, since 
they are generally not fertile anyway.

Good Luck,
Greta Hardin

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