Why are mules infertile?

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Offline chris

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Why are mules infertile?
« on: 19/12/2014 08:19:30 »
Someone was asking me why mules and other similar interbred species are frequently not fertile. What is the genetic mechanism for this?
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Offline CliffordK

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Re: Why are mules infertile?
« Reply #1 on: 19/12/2014 08:41:45 »
One of the mechanisms is that Donkeys and Horses have different chromosomes. 

In particular, the chromosome count:
Horses: 32 to 46
Donkey: 62
Horses: 64

See Wikipedia on Zebroids

So, a Donkey/Horse cross (mule) would have 63 chromosomes, 32 horse chromosomes and 31 donkey chromosomes.  Zebra crosses with donkeys or horses would be similar.  In a sense, it is surprising that they are even viable, but they obtain a complete half compliment of horse chromosomes, and a complete half compliment of donkey chromosomes.

Mitosis (normal cell division) simply requires dividing all the chromosomes, and giving each new cell a complete set of chromosomes. 

Meiosis (for reproduction) requires pairing up all the chromosomes and then dividing to give each cell half of the chromosomes.  Assuming it actually occurs in a mule, it would be interesting to see how it happens.  However, it would be difficult to get a complete haploid set of viable chromosomes.  Cross-overs might even scramble the chromosomes even more.

It would seem logical that a protection mechanism would have evolved to prevent mismatched chromosomes from generating viable gametes, although I haven't seen the research.