A mule is the offspring of a horse and a donkey. A donkey sperm contains 31 chromosomes and a horse egg has 32, so the zygote has 63. The zygote develops normally. However, a mule is sterile; meiosis cannot occur normally in its testes or ovaries. Explain why mitosis is normal in cells containing both horse and donkey chromosomes, but the mixed set interferes with meiosis.

Explanation:

The mules are the hybrid of the horse and donkey. The horse female gamete cell contains 32 chromosomes whereas the donkey sperm cell contains 31 chromosomes. The hybrid produced shows that the donkeys and horses shared the recent common ancestor.

The mules are infertile organisms as their gametes are not formed properly due to the difference in the number of chromosomes of the male and female gamete.

The gametes are produced by the process of meiosis which involves the pairing of homologous chromosomes but chromosomes fail to pair because of one extra chromosome which fails to for homologous pair.

The mitosis is possible in the organism as the mitosis process does not involve the pairing.