Why can mules not reproduce?

The horse has 64 chromosomes (32 pairs), and the donkey has 62 (31 pairs). When they are mated, the mule receives 63 chromosomes (32 from the horse and 31 from the donkey). During meiosis, the chromosomes are not in pairs, and, as their number is halved, the sex cells usually end up without a complete set. This unbalanced condition does not allow the sex cells to function, resulting in sterility.