A scinice question i dont understand?

A mule is the offspring of a horse and a donkey. A donkey sperm contains 31 chromosomes and a horse egg 32 chromosomes, so the zygote developes normally. The combined set of chromosomes is not the problem in mitosis, and the mule combines some of the best charcteristics of horses and donkeys. However, a mule is sterlile; meiosis cannot occur normally in its testes or ovaries. Explain why mitiosis is normal in cells contaning both horse and donkey chromosomes but the mixed set of chromosomes interferes with meiosis.