A mule is born, grows and lives a completely ordinary life, its cells dividing without any trouble. Yet almost none of them can have offspring. Where is the problem hiding?
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A mule results from crossing a male donkey with a mare. The donkey has 62 chromosomes and the horse 64, and the mule receives 63: 32 from the mare and 31 from the donkey. This odd total alone may not explain everything.
The blockage appears when sex cells are made. During this division, chromosomes from the mother and from the father must pair up. In mules, these pairs do not form correctly, and the cells produced are not viable. Researchers still debate the exact roles of the odd number and of the chromosomes being too different.
Exceptions exist, but they are rare: a few female mules have given birth to a foal, while no male mule has been recognised as a sire. Without an independent line, the cross must therefore be repeated each time.
