Ok, so when a horse (with 64 chromosomes) is crossed with a donkey(that has 62 chromosomes), each parent give its child half of its chromosomes. [64/2=32] [62/2=31]. So the mule gets 31 pairs of chromosomes plus 32 pairs of chromosomes. That equals 63 total chromosomes. In order to be a parent, it must give half of its chromosomes to its child. [63/2=31.5] You can't have half a chromosome, so the mule is a sterile organism. Let me know if you have questions.
