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I've read that female mules can sometimes have offspring, but that male mules are incapable of doing so. Why is that the case? Is their sperm not capable of entering the egg or is their a genetic reason?

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It's essentially genetic. The mule is a hybrid of a male donkey and female horse. A donkey has 62 chromosomes, a horse has 64 chromosomes, and a mule has 63 (mix of 62 and 64). There is a mismatch between both number and structure of the chromosomes and as a result, most mules are infertile.

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  • $\begingroup$ You say "most mules are infertile". Are you implying that some mules are fertile? $\endgroup$
    – James
    Oct 5, 2015 at 23:50
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    $\begingroup$ Yes, there are rare documented cases where mules have birthed foals. en.wikipedia.org/wiki/Mule#Fertility $\endgroup$
    – user560
    Oct 7, 2015 at 0:19
  • $\begingroup$ Bizarre and very interesting! $\endgroup$
    – James
    Oct 7, 2015 at 8:09

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