A mule will have 63 chromosomes, 31 from the donkey parent and 32 from the horse parent. This will leave unpaired chromosomes during meiosis, preventing successful gamete formation by the mule and thus lead to infertility.
Work Step by Step
The horse will have a 1n=32 (64/2) gamete which will join a 1n=31 (62/2) gamete from the donkey parent. This gives a zygote, and then a full-grown mule, with 32+31=63 chromosomes. Unpaired chromosomes in the mule will prevent meiosis from proceeding successfully, thus preventing gamete formation and reproduction by the mule.