Answer: Horses and donkeys have different numbers of chromosomes. Therefore, the offspring (mule) is unable to make sperm or eggs. This is because a horse and a donkey have different numbers of chromosomes. Horses have 64 and donkeys have 62. The baby gets half from the horse (32) and half from the donkey (31). When they combine they have a total of 63. During meiosis, when the chromosomes split, there is 31 pairs with 1 left over that cannot pair with another chromosome. This prevents all of the chromosomes from pairing so the sperm or eggs cannot be made.
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