What happens in the fertile mules that make them able to reproduce?
Just by chance, the right chromosomes/genes line up to give them working reproductive organs. Because there is a chromosomal mis-match between horses and donkeys that doesn’t usually happen, but every so often it does.
I got to Wikisurfing the other day (by way of the bedbugs thread) and came across a few even weirder ways to make offspring than mules (hybrids): parthenogenesisand gynogenesis. Bedbugs themselves are apparently obligate violent rapists. I had no idea such things existed.
But can you reproduce?
No, which is why work on the Resurrection Hub continues.