|MadSci Network: Genetics|
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 mammals 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 appear 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 half 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 functional 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 determine "true" sex, while a possibly interesting exercise, is not really useful, since they are generally not fertile anyway. Good Luck, Greta Hardin
Try the links in the MadSci Library for more information on Genetics.